How to Use the Ohm's Law Power Calculator
Ohm's Law describes the relationship between voltage, current, and resistance in a simple resistive circuit: V = I × R, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Combine that relationship with the definition of electrical power, P = V × I, and you can find the power dissipated by a component from any two of the three quantities — voltage, current, or resistance. Enter any two of the three inputs above and leave the remaining field blank; the calculator solves for the missing value with Ohm's Law and then computes power using whichever form of the power equation fits your inputs.
Deriving the three power equations
- P = V × I — the base definition of power: volts times amps equals watts. Use this when you know voltage and current.
- P = I²R — substitute Ohm's Law (V = IR) into P = VI to eliminate V. Use this when you know current and resistance.
- P = V²/R — substitute Ohm's Law (I = V/R) into P = VI to eliminate I. Use this when you know voltage and resistance.
All three forms are algebraically equivalent and always agree for a purely resistive load — the calculator simply picks the version that matches whichever two fields you filled in, then reports voltage, current, resistance, and power together so you can check the numbers against each other.
Worked example
- A 12 V supply pushes 2 A through a resistive load. Resistance: R = V ÷ I = 12 ÷ 2 = 6 Ω.
- Power: P = V × I = 12 × 2 = 24 W. Checking the other formulas: I²R = 2² × 6 = 24 W, and V²/R = 12² ÷ 6 = 24 W — all three match.
- These are the calculator's default values — leave Resistance blank and press Calculate to reproduce this example.
- Once you know the power, size the component with margin: a resistor dissipating 24 W should carry a rating of roughly 36-48 W (a 1.5-2× safety factor) for continuous duty.