Part of Circuit and resistor calculators
Calculate resistor dissipation from any two electrical values, then apply a design margin and select a standard power rating.
Select the electrical value to solve, then choose a design-margin multiplier.
Short pulses can exceed average power limits while still violating element voltage, pulse-energy, or overload ratings.
A resistor value can be electrically correct and still fail because it dissipates too much heat. Use this page to convert the circuit operating point into nominal dissipation, required margin, and a suggested standard wattage rating.
Check power when a resistor drops voltage or carries a known load current.
Check each voltage-divider resistor, especially when divider current is intentionally high.
Check small resistors used for current limiting, logic pull-ups, bleeders, and indicators.
Select the missing electrical value, enter the other two positive values, then choose a margin multiplier. Inputs accept engineering notation such as 10k, 4k7, 20m, 2.2M, and 47u.
Use when you know the voltage across the resistor and its resistance.
Use when you know resistor current and resistance.
Use when a measured or estimated operating point already gives voltage and current.
The calculator solves the resistor operating point with Ohm's law, calculates nominal power, multiplies by the requested margin, then suggests the next supported standard rating.
P = V × I
P = I² × R
P = V² / R
Required rating ≥ P × margin
Unit: watts (W)
Electrical power converted to heat by the resistor at the nominal operating point.
Unit: ratio
Design allowance applied to nominal power before choosing a suggested wattage rating.
Unit: watts (W)
A practical standard rating chosen above the calculated power times margin.
Unit: V, A, Ω
Voltage, current, and resistance values that must describe the same resistor condition.
The example below is checked against the same Ohm's law and resistor-rating helpers used by the calculator.
Design question: A 1 kΩ resistor has 10 V across it. What does it dissipate, and what standard rating is suggested with 2× margin?
Inputs: V = 10 V, R = 1 kΩ, margin = 2×.
Dissipation: P = V² / R = 10² / 1000 = 0.1 W = 100 mW.
Required with margin: 0.1 W × 2 = 0.2 W.
Suggested rating: the next listed standard rating above 0.2 W is 0.25 W.
Next check: confirm the resistor datasheet derating curve, ambient temperature, package size, maximum voltage, and whether the load is continuous or pulsed.
Calculated power tells you the nominal heat generated by the resistor. The selected wattage rating is a component choice that also depends on real operating conditions.
The basic equations assume a steady operating point. Pulsed loads need pulse-energy and overload checks.
Use worst-case voltage, current, and resistance values when the resistor is near its thermal or voltage limits.
A 0.25 W part in one package and layout is not automatically equivalent to another 0.25 W part in a hot enclosure.
Follow the next calculator or guide based on where the resistor power result came from.
Solve voltage, current, or resistance before checking resistor dissipation.
Use when the resistor is setting LED current and must also survive its power loss.
Use when two resistors share voltage and each leg needs a power check.
Follow related power, stored-energy, and conversion workflows.
Read why nominal dissipation is only the first wattage-selection check.
No. Calculated power is the nominal heat load. The selected resistor wattage should include design margin and must still be checked against datasheet derating, package size, ambient temperature, PCB copper, and pulse limits.
A 2× margin is a common first-pass screening value for benign continuous loads, but it is not universal. Hot enclosures, small packages, high voltage, pulse loads, and reliability requirements may need a larger margin or a dedicated thermal review.
Not always. Voltage rating, pulse energy, resistor technology, mounting style, PCB copper, and nearby heat sources can matter as much as the printed wattage value.
Engineering reference
Solves the DC Ohm law operating point and derives resistor dissipation with a user-selected power margin.
P = V * IP = V^2 / R = I^2 * RPrated >= P * marginInputs: V = 5 V, R = 100 ohm, margin = 2
Outputs: I = 50 mA, P = 250 mW, minimum with margin = 500 mW