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LED Series Resistor Calculator

Choose a standard resistor for a target LED current, or calculate current from an existing resistor. Include supply and LED voltage limits, switch drop, and resistor tolerance, then check the resulting current range and resistor power rating.

Help

Circuit values

Vin

Limits

Vf typ

Limits

MinMax

Use forward-voltage limits for the intended LED current and temperature.

N — LEDs in series

Target If

Ideal R

300.0 Ω

Include switch dropCurrent limits

Recommended standard R

Choose an E-series and tolerance, then compare the recommended value with nearby standard resistors.

E-series

Tolerance

Choose resistor to

Choose the resistor whose most distant current-range endpoint is closest to Target If.

Recommended value301.0 Ω, 1/10 W (100 mW)

Results use the selected resistor.

Resulting If range

Using R = 301.0 Ω, 298.0 Ω–304.0 Ω. Vin, Vf bounds contribute.

LED current range and signed deviation from target
Measurement	Min	Nominal	Max
Current	8.388 mA	9.967 mA	11.58 mA
Deviation from target (10 mA)	−16.12%	−0.33%	+15.78%

Choose a practical LED series resistor

Choose a standard resistor for a target LED current, or analyze the current from an existing resistor. Include voltage limits, an optional switch drop and resistor tolerance, then check current limits and power.

R = (Vin − N × Vf − Vsw) / If

Vin is the supply voltage; N is the number of identical LEDs in series; Vf is the forward drop per LED; Vsw is the total enabled switch or driver drop (zero when disabled); If is target current in amperes.

Worked example: exact resistance and current

For an exact 5 V supply, one 2 V LED, an enabled 0.2 V switch drop and 10 mA = 0.010 A, ideal R = (5 − 2 − 0.2) / 0.010 = 280 Ω.

To reproduce this exact operating point, select Solve for If, enter R = 280 Ω, and set every voltage and resistance variation to zero. Keep N = 1 and Include switch drop enabled with Vsw typ = 200 mV. The resulting current is 10 mA. These exact settings differ from Reset, which restores nonzero supply, LED-voltage and resistor variations.

Exact 280 Ω and 10 mA. Replaces current calculator inputs.

Power with exact 280 Ω and 10 mA

Result	Value
Resistor dissipation	28 mW
One LED	20 mW
Switch	2 mW
Supply	50 mW
Minimum nameplate / supported suggestion	56 mW / 1/16 W

The nominal power balance is 28 + 20 + 2 = 50 mW. With 50% utilization, 25 °C ambient, 70 °C full-rating and 125 °C zero-rating assumptions, the temperature factor is 100%, so 28 / (0.50 × 1.00) = 56 mW minimum nameplate power. The smallest supported rating meeting that requirement is 1/16 W.

Practical example: standard resistance and a current limit

Return to Solve for R with the same exact voltages, N = 1 and Target If = 10 mA. Select E24 and ±1% Tolerance. With Minimize worst-case current error, the recommendation is 270 Ω. To constrain the entire modeled range instead, enable current limits, leave If min blank, enter If max = 10 mA and select Keep full current range within limits: the recommendation becomes 300 Ω.

270 Ω accuracy objective. Replaces current calculator inputs.

Exact voltage drops, E24 and ±1% resistor tolerance

Objective / resistor	If min	If nominal	If max
Minimize worst-case current error / 270 Ω	10.268 mA	10.370 mA	10.475 mA
Keep full current range within limits / 300 Ω	9.241 mA	9.333 mA	9.428 mA

The 270 Ω option is closer to the nominal target but exceeds a 10 mA maximum. The full-window objective filters eligible values using the complete current interval before ranking them. A manual nearby selection can still violate your limits. Add the actual supply, Vf and switch bounds before accepting a design.

300 Ω with a 10 mA maximum. Replaces current calculator inputs.

PR = (Vin − N × Vf − Vsw)² / R

For that 270 Ω example, nominal resistor power is 29.04 mW and its modeled maximum is 29.33 mW. At the default 50% utilization, 25 °C ambient, 70 °C full-rating and 125 °C zero-rating assumptions, the required nameplate power is 58.66 mW; the next supported rating is 1/16 W. Check the actual component datasheet. To identify a selected part, read axial resistor color bands or decode an SMD resistor marking.

This is a fixed-Vf model for identical series LEDs, not a diode-curve or constant-current driver simulation. Parallel LED branches need independent current limiting. LED and driver electrical and thermal limits require separate checks.

How calculations are checked

Automated checks compare ideal resistance, fixed-drop current limits, standard-value objectives, nominal power balance and resistor derating with expected results. They also cover multiple LEDs, nonconducting headroom, current-limit warnings and invalid inputs. These checks verify the circuit model; they are not physical measurements. The About page describes the original spreadsheet and hand-calculation process.

Explore calculator features and benefits →

How to use

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Quick start

Choose a resistor — Solve for R

Select R under Solve for.
Enter Vin, Vf typ, N — LEDs in series, and Target If.
Set realistic voltage Limits; choose the E-series and resistor Tolerance.
Review Recommended standard R, Resulting If range, and Resistor power ratings.

Check a resistor — Solve for If

Select If under Solve for.
Enter Vin, Vf typ, N — LEDs in series, and the existing R.
Set voltage Limits and R limits/tolerance.
Review the current range and power rating.

Results update automatically; there is no Solve button. Optional settings may stay at defaults for an initial comparison. Default Vf bounds and current targets are examples, not verified ratings for your LED.

Calculator features and benefits

Choose or check a series resistor.
Compare standard values and evaluate worst-case current.
Recommend resistor wattage with utilization and temperature derating.

Reset restores the example inputs; it does not validate your LED design. See Reset and defaults below for the restored values.

Advanced features

Enter voltages, current and LED countShow

Vin spans the complete circuit. Vf typ and its bounds are per LED. N is a positive whole number of identical LEDs in series; all carry the same current.

Voltage entries 100 mv and 0.1 V both mean 100 mV. Current entries 10 ma, 10,000 uA, 10000 uA, and 0.01 A all mean 10 mA (10,000 µA). Bare voltage numbers mean V; bare current numbers mean mA. Fields reformat on blur or Enter. Resistance uses Ω, kΩ or MΩ.

A positive target current needs Vin nominal > N × Vf typ + Vsw typ. At 5 V with three 2 V LEDs, the 6 V LED drop exceeds the supply. More than two LEDs work with adequate voltage; there is no fixed two-LED limit. Worst-case bounds can include zero current even when nominal current is positive.

Set voltage/resistor limitsShow

Limits supports ± %, − / + %, ± value, or explicit Min / Max. Keep Min ≤ typical/nominal ≤ Max. Vf starts in Min / Max mode. Use realistic bounds from the actual device and operating conditions.

Choose resistor tolerance separately from E-series. In Solve for If, set the existing R limits; in Solve for R, the selected tolerance applies to recommended standard values.

LED forward voltage and color referenceShow

Example typical forward voltages by LED color

These are examples of typical Vf, not guaranteed minimum/maximum limits. Use your LED’s datasheet at the intended current and temperature.

LED color	Example typical Vf (V)	Notes
Red	1.65–2.25	GaAlAs at 2 mA (1.65 V); GaP at 20 mA (2.25 V).
Orange / amber	1.8–2.1	AlGaInP at 2 mA (1.8 V) and 20 mA (2.1 V).
Yellow	1.8–2.1	AlGaInP at 2 mA (1.8 V); GaAsP/GaP at 20 mA (2.1 V).
Yellow-green	1.8–2.25	AlGaInP low-current (2 mA) and older GaP (20 mA) examples.
True green	2.65–3.3	InGaN at 2 mA (2.65 V) and 20 mA (3.3 V).
Blue	2.65–3.8	AlGaInN at 2 mA (2.65 V); older GaN at 20 mA (3.8 V).
White	2.9–3.3	Different phosphor-converted devices at 5 mA and 20 mA.

Typical values represent different parts and technologies around 25 °C, not the bounds of a single device. A typical value alone cannot establish Min / Max design limits; obtain appropriate bounds for the actual part and conditions.

Sources: Kingbright technical data, pp. 73–77; white: APG1005VWF-A1-5MAV, 2.9 V at 5 mA and WP7113QWC/D, 3.3 V at 20 mA (both at 25 °C).

Include switch or output-driver dropShow

When Include switch drop is off, Vsw = 0 in the calculation. Turning it on exposes Vsw typ; 100 mV = 0.1 V. Initial limits are symmetric percentage at 0%. A 0 V nominal stays 0 V under percentage variation; use absolute tolerance or explicit bounds for a nonzero interval.

Saturated BJT: use V_CE(sat).
MOSFET: use a suitable I_f × R_DS(on) estimate.
Sinking output: use V_OL.
Sourcing output: use supply − V_OH when Vin denotes that supply.

These are fixed-drop estimates at the intended current, drive and temperature; the calculator does not solve the driver’s I-V characteristic. If Vin already denotes loaded output voltage, do not subtract the driver loss again. Check driver current and thermal ratings separately.

Choose and compare standard resistorsShow

All E-series combines E24, E48, E96 and E192 over 0.1 Ω–100 MΩ. Tolerance is separate. Compare nearby standard values shows alternatives; Use updates current, schematic and power. Recommended is the automatic choice; Selected is used in results. Changing an electrical input or selection setting chooses a new recommendation.

Objective	How the recommendation is chosen
Minimize worst-case current error	Minimizes the larger endpoint error relative to Target If; nominal current need not be the closest match.
Match target current at nominal values	Minimizes nominal current error; the full range can exceed entered limits.
Keep nominal current at or below target	Restricts nominal current only. Maximum current can exceed the target or entered limits.
Keep full current range within limits	Requires If max; If min is optional. Filters eligible standard values against the full window, then ranks by worst-case current error.

The first three objectives can recommend a value outside the entered current window; the calculator reports violations. A manually selected nearby alternative can also violate limits. Check its Resulting If range.

Open E-series reference below

Current limits and warningsShow

If min (optional) may be blank. If max is required for Keep full current range within limits. Other objectives report violations without constraining the recommendation. A failed minimum, maximum or both produces one detailed warning comparing each predicted endpoint with its limit.

Use design limits for your LED and conditions. An absolute maximum rating is not a universal recommended operating current. A nominal-at-or-below-target objective does not guarantee maximum current stays below target.

Detailed results and resistor power ratingsShow

Detailed results shows electrical values and nominal power allocation. Deviations use Target If in Solve for R and calculated nominal If in Solve for If. Resistor power ratings shows wattage, ambient temperature, utilization and derating. Check actual resistor mounting and datasheet conditions.

Review the maximum modeled resistor dissipation, then select realistic ambient temperature and utilization assumptions before choosing a component.

Open power-rating reference below

Troubleshooting unavailable resultsShow

Insufficient voltage: check Vin, N, Vf and Vsw against total drops; use suitable supply headroom or driver design.
Invalid input: correct the identified field; invalid text stays available for correction. Values must be finite, required positive quantities positive, N a positive whole number, and Min ≤ typical/nominal ≤ Max.
Current limit exceeded: compare endpoint and limit; review resistance, tolerance and realistic voltage bounds. Retain necessary design constraints.
No qualifying standard resistor: no supported value in the selected series within 0.1 Ω–100 MΩ meets the entered window. Review realistic limits, series/tolerance or circuit conditions. This differs from invalid input and insufficient voltage.
No suitable power rating: positive dissipation at zero temperature factor has no finite rating under this model; requirements may also exceed supported wattages.

Reset and defaultsShow

Reset restores the example configuration and closes expandable result sections. It does not establish suitability for any LED.

Setting	Default
Solve for / N	R / 1 LED
Vin	5 V ±5%
Vf typ / bounds	2 V / 1.8–2.2 V
Target If / existing R	10 mA / 300 Ω ±1%
E-series / tolerance	All E-series / ±1%
Objective	Minimize worst-case current error
Switch drop	Off; Vsw typ 0 V ±0%
Current limits	Off; If min and If max blank
Power assumptions	Ku 50%; Ta 25 °C; Tref 70 °C; Tzero 125 °C

Back to calculator ↑

Theory & formulas

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On narrow screens, scroll an equation sideways if needed to read all terms.

Series-resistor formulas

Variable	Meaning	Unit
Vin	Supply across complete circuit	V
Vf	Per-LED drop; Vf typ is the nominal control	V
N	Identical series LED count	whole number
Vsw	Total fixed switch/driver drop; zero when disabled	V
R	Series resistor	Ω
If	LED current	A
Ku / ku	Displayed utilization / fraction	% / none
Kt / kt	Displayed available power / fraction	% / none
Ta	Ambient temperature	°C
Tref	Full-power temperature	°C
Tzero	Zero-power temperature	°C

Use V, A, Ω and W in arithmetic: 10 mA = 0.01 A.

Rnom > 0 and target If > 0. Positive ideal R requires positive nominal headroom. Ideal R is the nominal target solution; results use the recommended or selected standard resistor. Vsw = 0 when disabled.

Current range

Tolerance t values are fractions: 1% = 0.01. Absolute tolerances and explicit limits define Rmin and Rmax directly.

Independent worst-case bounds are not statistical confidence intervals. Every series LED may reach the entered per-LED extreme.

The reference is Target If in Solve for R and calculated nominal If in Solve for If. Positive means above reference; negative means below. No percentage is shown for an unavailable or nonpositive reference.

Selection objectives

Objective	How the recommendation is chosen
Minimize worst-case current error	Minimizes the larger endpoint error relative to Target If; nominal current need not be the closest match.
Match target current at nominal values	Minimizes nominal current error; the full range can exceed entered limits.
Keep nominal current at or below target	Restricts nominal current only. Maximum current can exceed the target or entered limits.
Keep full current range within limits	Requires If max; If min is optional. Filters eligible standard values against the full window, then ranks by worst-case current error.

Power ratings

Component	Electrical power (W)
Resistor	PR = If² × R
All series LEDs	PLED,string = If × N × Vf
One LED	PLED,per LED = If × Vf
Switch / driver drop	Psw = If × Vsw
Supply	Pin = Vin × If

Use one consistent conducting operating point, usually nominal. Component powers then sum to supply power; independent range extrema need not represent that same condition. LED electrical power includes heat and light and does not predict brightness.

Do not mix If max with an unrelated R max or R nominal.

For wattage selection, utilization margin and temperature derating, use the complete reference below.

Open power-rating reference below

Assumptions and practical limits

Fixed forward-drop estimates; not a diode-curve or brightness solver. Vf and driver drops depend on current, temperature and device variation; revisit estimates if current changes substantially.
Zero current is a headroom outcome, not a leakage calculation.
One Vf specification models identical series LEDs; mixed devices are not individually modeled.
Give parallel LED branches separate current limiting.
Continuous calculations do not establish pulse capability.
Check LED ratings, driver limits, resistor working voltage, and thermal/mounting conditions. Not every LED is rated for 20 mA.

A constant-current driver can improve regulation for wide voltage variation or high current. Linear drivers dissipate excess voltage; switching drivers can reduce this loss.

Back to calculator ↑

E-series resistor values

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E-series are standardized sets of preferred nominal resistance values defined by IEC 60063. They provide a consistent selection of values for circuit design and component manufacturing.

The nominal resistance is the specified value of a resistor. Its actual resistance may differ within its stated tolerance. The E-series identifies a set of nominal values; tolerance describes the permitted variation around a selected value.

Values per decade

The number following E indicates how many unique nominal values the series contains per decade. A decade spans a factor of ten, such as 100 Ω to below 1 kΩ or 1 kΩ to below 10 kΩ.

E6, for example, contains these six values between 1 kΩ and 10 kΩ: 1.0 kΩ, 1.5 kΩ, 2.2 kΩ, 3.3 kΩ, 4.7 kΩ, and 6.8 kΩ.

The pattern repeats in other decades by multiplying or dividing by powers of ten. The 10 kΩ boundary begins the next decade, so it is not counted twice.

Series and commonly associated tolerances

Series	Values per decade	Significant digits	Commonly associated tolerances
E3	3	2	No tolerance association listed in the cited Vishay table
E6	6	2	±20%
E12	12	2	±10%
E24	24	2	±5%
E48	48	3	±2%
E96	96	3	±1%
E192	192	3	±0.5%, ±0.25%, ±0.1%

These are conventional associations, not limits on the tolerances available for each value. Manufacturers offer preferred values at multiple tolerances, depending on the product family, resistance range, and package. Check the component datasheet for available combinations.

For example, a nominal 10 kΩ resistor with ±1% tolerance has resistance limits of 9.90 kΩ and 10.10 kΩ under the specified datasheet conditions.

R_min = R_nominal × (1 − t) R_max = R_nominal × (1 + t)

Use 0.01 for ±1%. E3, E6, E12, and E24 form one nested group. E48, E96, and E192 form another. E24 is not fully contained in E192: for example, E24 includes 2.2 kΩ, while E192 includes nearby 2.21 kΩ.

All E-series

All E-series searches every preferred resistance value in E24, E48, E96, and E192, with duplicates removed. This also includes E3, E6, and E12 values.

The combined set contains 205 unique values per decade. It is formed by combining E24 and E192, which together cover all the series listed above.

Choose this option for the broadest selection of values supported by the calculator, then check your distributor for components with the required tolerance, ratings, availability, and price. If a suggested component is unavailable or unsuitable, evaluate another suggested value or an available value from the distributor’s catalog.

All E-series is a calculator search category, not a separate standardized E-series. It does not include every custom or nonstandard resistance sold by manufacturers, and it does not guarantee that a particular component is stocked.

Choosing preferred values

Choose All E-series when you want a broad search and plan to source components from a distributor. Choose an individual series when you want to match an existing inventory or a manufacturer’s product range.

Select tolerance separately. The preferred-value set determines which nominal resistances are considered; tolerance determines the resistance variation used in the calculations.

A closer nominal match does not necessarily produce the best worst-case circuit performance. Compare the calculated circuit results with the selected values and tolerances, as well as the component ratings.

A practical resistor-selection workflow

Define the circuit requirements. Establish the desired resistance or circuit output, acceptable error, operating conditions, and relevant size, voltage, and power constraints.
Calculate the ideal value. Determine the resistance that would produce the desired result with ideal components.
Select preferred values and tolerance. Choose All E-series for a broad search, or an individual series to match your sourcing requirements. Set the tolerance you intend to purchase.
Compare circuit performance. Review nominal and worst-case results using the suggested values. Check resistor power dissipation and the recommended power rating. For resistor combinations, evaluate the complete circuit as well as each resistor.
Find suitable components. Search a distributor’s catalog for the selected resistance and tolerance. Check package, rated power and derating, temperature coefficient, voltage rating, stock, lead time, and price at your required quantity. Check pulse capability when applicable.
Evaluate alternatives when necessary. If a component is unavailable or unsuitable, consider another suggested value or enter an available value into the calculator. Recalculate with the tolerance of the actual component you intend to buy.
Specify the selected component. Record the manufacturer’s part number and relevant specifications in the bill of materials. Verify the datasheet ratings and operating conditions before finalizing the design.

The goal is to select components that meet the circuit requirements and can be sourced at an acceptable price. The closest nominal resistance is a useful starting point; the final selection also depends on tolerance, operating limits, and availability.

E-series and component price

A larger E-series does not automatically mean a higher component price. Different resistance values within the same product family may have identical or similar prices, while particular values can differ in price, stock, or lead time.

Compare actual components with the same relevant specifications, purchase quantity, and packaging. Use the calculator to evaluate electrical performance and the distributor’s catalog to confirm purchasing options.

How preferred values are spaced

E-series values follow approximately equal percentage steps rather than equal differences in ohms. For a series with N values per decade, the ideal adjacent-value ratio is 10^1/N.

Ideal step = 100 × (10^1/N − 1)%

This is approximately 10.1% for E24 and 2.43% for E96. Actual tabulated steps vary because the standardized values use rounded numbers.

Use the published preferred-value tables when selecting components. The formula explains the approximate spacing; it does not replace the standardized tables.

Preferred-value tables

Each table lists every preferred value within one decade, normalized from 1 up to—but not including—10. Multiply an entry by a power of ten to obtain resistance values in other decades. For example, 4.7 represents 4.7 Ω, 47 Ω, 470 Ω, 4.7 kΩ, 47 kΩ, and so on. Inclusion in a table does not guarantee availability in a particular tolerance or package.

E3 — 3 values per decade

E6 — 6 values per decade

E12 — 12 values per decade

E24 — 24 values per decade

E48 — 48 values per decade

1.00
1.05
1.10
1.15
1.21
1.27
1.33
1.40
1.47
1.54
1.62
1.69
1.78
1.87
1.96
2.05
2.15
2.26
2.37
2.49
2.61
2.74
2.87
3.01
3.16
3.32
3.48
3.65
3.83
4.02
4.22
4.42
4.64
4.87
5.11
5.36
5.62
5.90
6.19
6.49
6.81
7.15
7.50
7.87
8.25
8.66
9.09
9.53

E96 — 96 values per decade

1.00
1.02
1.05
1.07
1.10
1.13
1.15
1.18
1.21
1.24
1.27
1.30
1.33
1.37
1.40
1.43
1.47
1.50
1.54
1.58
1.62
1.65
1.69
1.74
1.78
1.82
1.87
1.91
1.96
2.00
2.05
2.10
2.15
2.21
2.26
2.32
2.37
2.43
2.49
2.55
2.61
2.67
2.74
2.80
2.87
2.94
3.01
3.09
3.16
3.24
3.32
3.40
3.48
3.57
3.65
3.74
3.83
3.92
4.02
4.12
4.22
4.32
4.42
4.53
4.64
4.75
4.87
4.99
5.11
5.23
5.36
5.49
5.62
5.76
5.90
6.04
6.19
6.34
6.49
6.65
6.81
6.98
7.15
7.32
7.50
7.68
7.87
8.06
8.25
8.45
8.66
8.87
9.09
9.31
9.53
9.76

E192 — 192 values per decade

1.00
1.01
1.02
1.04
1.05
1.06
1.07
1.09
1.10
1.11
1.13
1.14
1.15
1.17
1.18
1.20
1.21
1.23
1.24
1.26
1.27
1.29
1.30
1.32
1.33
1.35
1.37
1.38
1.40
1.42
1.43
1.45
1.47
1.49
1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
1.65
1.67
1.69
1.72
1.74
1.76
1.78
1.80
1.82
1.84
1.87
1.89
1.91
1.93
1.96
1.98
2.00
2.03
2.05
2.08
2.10
2.13
2.15
2.18
2.21
2.23
2.26
2.29
2.32
2.34
2.37
2.40
2.43
2.46
2.49
2.52
2.55
2.58
2.61
2.64
2.67
2.71
2.74
2.77
2.80
2.84
2.87
2.91
2.94
2.98
3.01
3.05
3.09
3.12
3.16
3.20
3.24
3.28
3.32
3.36
3.40
3.44
3.48
3.52
3.57
3.61
3.65
3.70
3.74
3.79
3.83
3.88
3.92
3.97
4.02
4.07
4.12
4.17
4.22
4.27
4.32
4.37
4.42
4.48
4.53
4.59
4.64
4.70
4.75
4.81
4.87
4.93
4.99
5.05
5.11
5.17
5.23
5.30
5.36
5.42
5.49
5.56
5.62
5.69
5.76
5.83
5.90
5.97
6.04
6.12
6.19
6.26
6.34
6.42
6.49
6.57
6.65
6.73
6.81
6.90
6.98
7.06
7.15
7.23
7.32
7.41
7.50
7.59
7.68
7.77
7.87
7.96
8.06
8.16
8.25
8.35
8.45
8.56
8.66
8.76
8.87
8.98
9.09
9.20
9.31
9.42
9.53
9.65
9.76
9.88

All E-series — 205 values per decade

Combined E24 and E192 values, with duplicates removed.

1.00
1.01
1.02
1.04
1.05
1.06
1.07
1.09
1.10
1.11
1.13
1.14
1.15
1.17
1.18
1.20
1.21
1.23
1.24
1.26
1.27
1.29
1.30
1.32
1.33
1.35
1.37
1.38
1.40
1.42
1.43
1.45
1.47
1.49
1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
1.65
1.67
1.69
1.72
1.74
1.76
1.78
1.80
1.82
1.84
1.87
1.89
1.91
1.93
1.96
1.98
2.00
2.03
2.05
2.08
2.10
2.13
2.15
2.18
2.20
2.21
2.23
2.26
2.29
2.32
2.34
2.37
2.40
2.43
2.46
2.49
2.52
2.55
2.58
2.61
2.64
2.67
2.70
2.71
2.74
2.77
2.80
2.84
2.87
2.91
2.94
2.98
3.00
3.01
3.05
3.09
3.12
3.16
3.20
3.24
3.28
3.30
3.32
3.36
3.40
3.44
3.48
3.52
3.57
3.60
3.61
3.65
3.70
3.74
3.79
3.83
3.88
3.90
3.92
3.97
4.02
4.07
4.12
4.17
4.22
4.27
4.30
4.32
4.37
4.42
4.48
4.53
4.59
4.64
4.70
4.75
4.81
4.87
4.93
4.99
5.05
5.10
5.11
5.17
5.23
5.30
5.36
5.42
5.49
5.56
5.60
5.62
5.69
5.76
5.83
5.90
5.97
6.04
6.12
6.19
6.20
6.26
6.34
6.42
6.49
6.57
6.65
6.73
6.80
6.81
6.90
6.98
7.06
7.15
7.23
7.32
7.41
7.50
7.59
7.68
7.77
7.87
7.96
8.06
8.16
8.20
8.25
8.35
8.45
8.56
8.66
8.76
8.87
8.98
9.09
9.10
9.20
9.31
9.42
9.53
9.65
9.76
9.88

Sources

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Resistor power ratings and derating

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What the rating means

A resistor converts electrical power into heat. Its dissipation is the power produced by the circuit; its rated or nameplate wattage is the manufacturer's allowed dissipation under specified temperature and mounting conditions. A 1/4 W resistor does not always dissipate 1/4 W, and its full rating may not be available in a hot enclosure or with different mounting conditions.

For one operating point, resistor dissipation can be calculated as P = V_R × I = I² × R = V_R² / R, where V_R is the voltage across that resistor. Use volts, amperes, ohms, and watts in the equations: 10 mA = 0.010 A and 100 mW = 0.100 W. Evaluate each resistor separately; total network power does not determine each resistor's rating.

CircuitKitchen determines a power basis for each resistor, applies utilization margin and temperature derating, and selects the smallest supported wattage that meets the requirement. The suggestion is conditional on the entered circuit bounds and selected assumptions.

Determine the power basis

For continuous operation, P_basis is that resistor's maximum modeled continuous dissipation, P_max. Use the selected or entered resistor values and their limits, rather than only the ideal resistance or nominal circuit power.

Voltage Divider evaluates Rtop and Rbot separately over the entered supply, resistor, and applicable load limits. Its model also considers relevant intermediate resistance values that can produce a maximum between tolerance endpoints. Use the calculated Pmax for each resistor.

Multi-Tap evaluates each ladder resistor separately in complete combinations of supply and resistor-tolerance endpoints. At each evaluated state, P_i = I_ladder² × R_i. The reported maximum for each resistor supplies its rating calculation. The maximum-power states for different resistors need not be the same. Do not imply that this model searches intermediate resistance extrema or that it models loaded taps.

Parallel evaluates each branch at the specified voltage across the network. With a fixed voltage, P_i = V² / R_i, so the lowest resistance in that branch's tolerance range produces its highest power. Unequal resistances do not share power equally.

LED uses the series resistor's maximum modeled dissipation, accounting for supply limits, LED count and forward-voltage limits, enabled switch/driver drop, and resistor limits. In the fixed-drop model:

Vf is per LED; Vsw is the total enabled switch/driver drop and is zero when disabled. Use a consistent operating state. Do not combine If_max with an unrelated R_max to calculate power. These fixed-drop estimates do not model diode curves or changes in Vf with temperature automatically.

For Parallel's repeating rectangular pulses, use worst-case average power as the thermal basis:

Duty cycle D is a fraction. A 25% duty cycle is D = 0.25. The other calculators do not provide this pulse-rating calculation.

Temperature derating

Temperature derating reduces the fraction of nameplate wattage available at the selected temperature. CircuitKitchen uses a generic linear approximation:

T_a: highest expected local ambient temperature near the resistor.
T_ref: temperature through which the model allows full rated power.
T_zero: temperature at which the model allows zero power.
k_t: available fraction of nameplate power.

T_zero must exceed T_ref. Use finite, valid settings. When dissipation is positive and k_t = 0, increasing wattage cannot produce a suitable finite rating under this model; review the operating conditions or the appropriate component thermal model.

Use the highest expected local temperature, including enclosure heating, airflow, nearby components, and PCB heating. T_a is not automatically the resistor body's operating temperature. Some datasheets base ratings on terminal, case, or board temperatures and specific cooling conditions. Do not substitute those temperatures directly into an ambient model without establishing that the thermal assumptions apply. This calculator does not solve component self-heating or a complete thermal network.

Utilization margin

The utilization setting limits modeled dissipation to a chosen fraction of the temperature-adjusted rating. Lower utilization provides more margin but requires greater nameplate wattage. Use the percentage shown in the control as a fraction in arithmetic: k_u = Ku_percent / 100. A 50% setting means k_u = 0.50. Similarly, a displayed Kt of 100% means k_t = 1.00; some panels display Kt directly as a fraction.

These equations apply with positive valid k_u and k_t. A smaller utilization setting and a higher ambient temperature can independently increase the required rating. Utilization and temperature derating address different assumptions; the recommended wattage already includes both. Do not apply the same selected factors a second time.

Select a supported wattage

Choose the smallest supported rating equal to or above P_rated,min, never a lower rating. CircuitKitchen's supported list currently runs from 1/32 W through 5 W and includes intermediate ratings; use the calculator's actual list rather than assuming every step doubles. A suggested wattage is not a guarantee that a particular resistance, tolerance, package, and wattage combination is sold or stocked.

If the required rating exceeds the supported list, no listed rating meets the requirement. This differs from invalid input and from zero available power at the selected temperature. Review the circuit, cooling, component family, or required rating. For an LED state with no modeled resistor dissipation, preserve its "No dissipation predicted" result; this does not establish suitability under unmodeled conditions.

Defaults and worked examples

CircuitKitchen defaults are T_a = 25 °C, Ku = 50%, T_ref = 70 °C, and T_zero = 125 °C. They are initial calculator assumptions, not universal resistor specifications or a guarantee of safety. Choose settings appropriate to the actual component and operating environment.

At the defaults, k_t = 1.00 and k_u = 0.50. For P_basis = 100 mW:

The next supported wattage is 1/4 W, or 250 mW.

At T_a = 85 °C with the same T_ref, T_zero, utilization, and dissipation:

The next supported wattage is 1/3 W. Higher ambient temperature requires a higher rated component even though circuit dissipation has not changed.

For a Parallel pulse example, P_peak,worst = 1 W and D = 0.10 give P_basis = 0.10 W. At the default factors the minimum continuous rating is 0.20 W, suggesting 1/4 W thermally. This result alone does not establish pulse survivability.

Pulse limits and final component selection

A resistor may meet its average-power requirement and still exceed peak power, pulse width, pulse energy, repetition rate, repetitive pulse, working-voltage, or overload limits. For a rectangular pulse, energy per pulse is E = P_peak × t_on in joules. Check the manufacturer's pulse curves and conditions separately; do not infer allowable pulses from continuous wattage alone.

Use this workflow: review circuit bounds and each resistor's power basis; select realistic temperature and utilization assumptions; check the suggested wattage; then source an actual component and verify its datasheet. Check power and derating, mounting and PCB thermal conditions, maximum working voltage, overload and pulse limits, package, resistance, tolerance, and temperature coefficient. Verify thermal behavior in the intended assembly when needed. Keep LED and driver electrical and thermal limits separate from the resistor rating check.

Derating changes allowed dissipation. It does not model resistance drift, temperature coefficient, aging, or LED forward-voltage changes. A recommended rating is a design aid under the selected model, not a substitute for component specifications or thermal validation.

Sources

General rating and pulse guidance; the selected component’s datasheet controls its specifications.

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Resistor power ratings and derating

What the rating means

Determine the power basis

Temperature derating

Utilization margin

Select a supported wattage

Defaults and worked examples

Pulse limits and final component selection

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