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Parallel Resistor Calculator

Design parallel resistor networks, not just calculate them. Enter 2–10 resistors, find standard-value combinations for a target, and verify tolerance, current, power, derating, and pulse loading.

Move from a target resistance to a practical resistor network and power check without repeating hand calculations or building a spreadsheet.

Help

Calculate combined resistance

Enter 2–10 resistor values to calculate the nominal combined resistance and its tolerance range.

Combined resistance6.000 kΩ

Range: 5.940 kΩ – 6.060 kΩ

Find Resistors for a Target

Search standard resistor combinations by E-series, tolerance, and resistor count—using exactly N or up to N resistors.

Target resistance

E-seriesTolerance

Resistor count

resistors

Search range: each resistor must be greater than the target and no more than 10× the target.

Parallel resistance, tolerance and branch power

Analyze an existing 2–10 resistor network or find standard-value combinations for a target. Review nominal resistance, tolerance, branch current, power and wattage for the selected network.

Analyze a known pair

Enter R1 = 10 kΩ and R2 = 15 kΩ, each at ±1%. Their parallel resistance is (10 × 15) / (10 + 15) = 6 kΩ; the limits are 5.94–6.06 kΩ.

Known parallel pair. Replaces current calculator inputs.

Choose a pair for a target resistance

Under Find Resistors for a Target, enter 7.330 kΩ, choose E96, ±1% and Exactly 2 resistors, then select Find combinations. The top pair is 8.87 kΩ ∥ 42.2 kΩ. Select Use to apply it to the network. Loading this example performs the search, applies its top pair and opens the 5 V continuous-power results.

Nominal combined resistance is approximately 7.32943 kΩ, an error of −0.00777%. Its ±1% manufacturing limits are approximately 7.25614–7.40272 kΩ. The tiny nominal selection error does not replace component tolerance.

Open Power and current and enter Network voltage = 5 V with Continuous operation. Nominal branch currents are 0.5637 mA and 0.1185 mA; nominal branch powers are 2.818 mW and 0.5924 mW. At a fixed 5 V, the minimum resistance in each ±1% range gives worst-case powers of approximately 2.847 mW and 0.5984 mW.

E96 pair selection and power. Replaces current calculator inputs.

Every branch sees the full network voltage, so unequal resistors do not share current or power equally. This page accepts network voltage and calculates current; it does not provide a constant-current source input. Check the selected parts’ working voltage and thermal conditions separately.

Applicable two-resistor searches evaluate the available pairs; searches with more resistors are bounded and do not guarantee a global optimum. For repeating pulses, average power is a thermal estimate, while peak power, pulse duration, energy and datasheet pulse limits still govern survivability.

How calculations are checked

Automated checks cover equivalent resistance and limits, standard-value search and ranking, branch current and power, pulse timing and energy, derating, and invalid or boundary inputs. The About page describes the original spreadsheet and hand-calculation process. These checks verify the circuit model; they are not physical component measurements.

Explore calculator features and benefits →

To identify a selected component, read axial resistor color bands or decode an SMD resistor marking.

How to use

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Calculator features and benefits

CircuitKitchen can be used as a complete parallel-resistor selection workflow. Start with resistor values you already have, or start with a target resistance and let the calculator find practical standard-value combinations. Then evaluate tolerance, current, power, resistor wattage, and continuous or pulsed operation without moving the design into a separate spreadsheet.

Analyze a network — Enter 2–10 parallel resistors to calculate combined resistance and the range produced by resistor tolerance.
Find standard values for a target — Search the selected E-series and tolerance using exactly the selected number of resistors or any count from 2 up to that number.
Check resistor power — Calculate branch current and worst-case dissipation and get a recommended standard wattage using CircuitKitchen's utilization and temperature-derating model.
Evaluate pulsed loads — Enter on-time and either period or frequency to calculate duty cycle, peak power, average power, and energy per pulse.

Calculate combined resistance

Enter the resistor values connected in parallel. The calculator supports 2–10 resistors and shows both the nominal combined resistance and the minimum/maximum range produced by the entered resistor tolerances.

Use this mode when you already have candidate resistor values and want to evaluate the resulting network.

Tip: Combined resistance is always less than the smallest resistor in the parallel network.

Find resistors for a target

Enter the target combined resistance you want to build, then choose the resistor-value series, tolerance, and resistor count.

Exactly searches combinations containing exactly the selected number of resistors. Up to searches combinations using 2 through the selected maximum.

The calculator compares practical standard-resistor combinations and reports their combined resistance, nominal error, and tolerance error so you can compare real component choices instead of manually trying values.

Select Use to load a result into Calculate combined resistance. The selected network can then be evaluated immediately for resistance range, current, power, and resistor rating.

Two-resistor searches evaluate every applicable pair. Searches using more than two resistors use a bounded search to remain responsive, so the displayed combinations are the best found within that search rather than proof of a global optimum.

Search range: target resistance < resistor value ≤ 10× target resistance.

Open E-series reference below

Check power and resistor ratings

Enter the voltage across the parallel network to calculate current and power for the complete network and for each resistor individually.

CircuitKitchen evaluates resistor tolerance when determining worst-case power and recommends a standard resistor wattage using the selected design margin and temperature assumptions.

The rating model uses:

Ta — maximum expected ambient temperature near the resistor
Ku — utilization factor used to provide design margin
Kt — temperature factor calculated from Ta, Tref, and Tzero
Tref — reference temperature through which the generic model allows full rated power
Tzero — temperature where the generic derating model reaches zero allowable power

The defaults provide a conservative starting point and can be adjusted for the resistor family, operating environment, and design policy.

Open power-rating reference below

Continuous loads

Select Continuous when the network voltage is continuously applied.

The calculator shows branch current, nominal and worst-case resistor power, total network power, and a recommended standard power rating for each resistor.

Pulsed loads

Select Pulsed for a repeating rectangular voltage pulse.

Enter Ton and either Period or Frequency. The calculator determines the corresponding timing values and calculates:

duty cycle
pulse current
worst-case peak resistor power
average resistor power
energy per pulse
average total network power

The recommended continuous resistor wattage uses the worst-case average power as its thermal basis and applies the same utilization and temperature-derating assumptions used for continuous operation.

Pulse capability must still be verified separately. A resistor can meet its average wattage requirement and still be damaged by excessive peak power, pulse width, pulse energy, repetitive pulse loading, working voltage, or overload voltage. Check the manufacturer's pulse and overload specifications.

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Theory & formulas

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Combined resistance

Combined resistance, also called equivalent resistance R_eq, is the single resistance that has the same electrical behavior as all of the parallel resistors together.

For any number of resistors:

\frac{1}{R_{eq}} = \frac{1}{R_{1}} + \frac{1}{R_{2}} + \frac{1}{R_{3}} + \dots + \frac{1}{R_{n}}

or:

R_{eq} = \frac{1}{(\frac{1}{R_{1}} + \frac{1}{R_{2}} + \frac{1}{R_{3}} + \dots + \frac{1}{R_{n}})}

For two resistors:

R_{eq} = \frac{R_{1} R_{2}}{R_{1} + R_{2}}

For N identical resistors, each with resistance R:

R_{eq} = \frac{R}{N}

Quick check

For resistors in parallel:

R_{eq} < R_{smallest}

The combined resistance must always be less than the smallest individual resistor.

Voltage and current

Every resistor in a parallel network has the same voltage across it.

The current through each resistor is:

I_{i} = \frac{V}{R_{i}}

Total current is the sum of the branch currents:

I_{total} = I_{1} + I_{2} + \dots + I_{n}

It can also be calculated from the combined resistance:

I_{total} = \frac{V}{R_{eq}}

A lower-value resistor carries more current than a higher-value resistor connected across the same voltage.

Resistor power

Power dissipated by each resistor can be calculated using:

P_{i} = V I_{i} = \frac{V^{2}}{R_{i}} = {I_{i}}^{2} R_{i}

Total network power is:

P_{total} = P_{1} + P_{2} + \dots + P_{n} = \frac{V^{2}}{R_{eq}}

Because all parallel resistors see the same voltage, a lower resistance dissipates more power than a higher resistance at that voltage.

Do not assume the total power is divided equally among the resistors unless their resistance values are equal.

Open power-rating reference below

Tolerance and combined resistance

Real resistors vary from their nominal values according to their specified tolerance.

The minimum combined resistance occurs when the branch resistances are at their applicable minimum values. The maximum combined resistance occurs when they are at their applicable maximum values.

The calculator reports the resulting combined-resistance range.

For a target resistance, nominal error is:

Nominal error = \frac{R_{combined} - R_{target}}{R_{target}} \times 100 %

The tolerance error range shows how far the combined resistance could vary from the target after resistor tolerances are included.

Standard resistor values

Commercial resistors are commonly manufactured in standardized E-series values such as E24, E48, E96, and E192.

The target search uses values from the selected series rather than arbitrary resistance values.

Open E-series reference below

Pulsed operation

For a repeating rectangular pulse:

D = \frac{t_{on}}{T}

where D is duty cycle, t_on is pulse on-time, and T is pulse period.

f = \frac{1}{T}

P_{avg} = P_{peak} D

E_{pulse} = P_{peak} t_{on}

Average power is useful for evaluating thermal loading, but it does not by itself establish pulse survivability. Peak power, pulse width, pulse energy, repetition rate, working voltage, and the resistor’s pulse rating must also be checked.

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Design tips

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Check each resistor individually

Parallel resistors do not necessarily share current or power equally.

lower resistance → higher current
lower resistance → higher power dissipation

Check the current, power, and rating of every resistor rather than considering only total network power.

Remember that every resistor sees the full voltage

Putting resistors in parallel does not divide the applied voltage.

Each resistor sees essentially the full voltage across the parallel network. Always verify the resistor’s maximum working voltage in addition to its power rating.

Use equal values when equal power sharing is desirable

Identical resistor values nominally divide current and power equally.

For N identical resistors, , and each resistor nominally carries of the total current and dissipates approximately of the total resistor power.

Actual sharing can vary with resistor tolerance and temperature.

Consider whether the extra resistor is worthwhile

Adding more resistors can produce a closer match to a target resistance and distribute power among more components, but it also adds:

component count
PCB area
assembly cost
additional tolerance contributors
additional possible failure points

When two solutions provide nearly the same accuracy, the simpler solution may be preferable.

Consider tolerance, not just nominal error

A combination with an extremely small nominal target error is not necessarily the best practical design if resistor tolerance produces a much wider possible range.

Compare both Nominal error and Tolerance error. Choose values appropriate for the accuracy the circuit actually requires.

Leave power margin

Avoid selecting a resistor solely because its nominal wattage barely exceeds calculated dissipation.

CircuitKitchen can apply utilization margin and temperature derating when recommending a rating.

Open power-rating reference below

Also check the selected resistor’s actual datasheet for its derating curve, mounting requirements, PCB thermal conditions, maximum working voltage, overload limits, and pulse capability.

Account for local temperature

The temperature near the resistor may be significantly higher than room temperature because of nearby power components, PCB copper temperature, restricted airflow, enclosure temperature, and heat generated by adjacent resistors.

Use a realistic maximum ambient temperature near the resistor, not simply the expected room temperature.

Verify pulsed loads separately

A resistor can satisfy its average-power requirement and still be damaged by a short, high-power pulse.

For pulsed designs, verify the manufacturer’s limits for peak power, pulse width, pulse energy, repetition rate, repetitive pulse operation, and maximum working and overload voltage.

Do not infer pulse capability from the continuous wattage rating alone.

Use the calculator as a design aid

The calculator helps evaluate resistance, tolerance, current, power, and standard component choices, but the final component selection should be verified against the resistor manufacturer’s specifications and the requirements of the actual circuit.

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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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