Resistor Power Dissipation Calculator
Every resistor turns electrical energy into heat as current passes through it. Knowing how much power a resistor dissipates lets you pick a part with an adequate power rating so it does not overheat or fail. The dissipated power follows directly from Ohm's law and can be found from any two of voltage, current, and resistance. Enter the voltage across the resistor and the current through it; this calculator returns the resistance, the dissipated power computed three equivalent ways, and a suggested minimum power rating with a safety margin.
Power dissipation formula
Power = voltage * current
Power = current^2 * resistance
Power = voltage^2 / resistance
Resistance = voltage / current
Suggested rating = power * safety margin
Voltage is in volts, current in amperes, resistance in ohms, and power in watts. All three power formulas give the same value for a resistor obeying Ohm's law.
Resistor heating context
- Power dissipation is the rate of Joule heating; it equals voltage times current for a resistor.
- Common axial resistor ratings are 0.125, 0.25, 0.5, 1, and 2 watts; choose one above the dissipated power.
- A safety margin of about two leaves headroom for ambient heat, airflow, and tolerance.
- Exceeding the rated power causes the resistor to overheat, drift in value, or burn out.
- Surface-mount resistors have power ratings tied to their package size and the board's heat-sinking.
Power dissipation: frequently asked questions
How do I calculate the power dissipated in a resistor?
Power dissipated equals voltage times current (P = V x I). Using Ohm's law you can also write it as P = I^2 x R or P = V^2 / R. All three give the same result for a resistor and let you compute power from whichever quantities you know.
How do I size a resistor's power rating?
Choose a resistor whose power rating comfortably exceeds the calculated dissipation. A common rule of thumb is to use a rating at least twice the expected dissipation to allow a safety margin for heat, ambient temperature, and tolerance. The rating choice is yours; this tool gives the dissipated power.
What units does this use?
Voltage is in volts, current in amperes, resistance in ohms, and power in watts. These are coherent SI units, so no conversion is needed: one volt across one ohm draws one ampere and dissipates one watt.
Why does a resistor heat up?
A resistor converts electrical energy into heat as current flows through it, a process called Joule heating. The power dissipated equals the rate of heat generation. If this exceeds the resistor's power rating, it can overheat, drift in value, or fail.
Which formula should I use?
Use the formula that matches the quantities you know. This calculator computes resistance from the voltage and current you enter (R = V / I), then reports power three ways so you can cross-check. Enter the actual operating voltage and current for your circuit.
Official sources
- NIST: SI electrical units, the watt, volt and ohm (SP 811).
- NIST: Physical Measurement Laboratory (electrical metrology).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.