Ohm's Law calculator
Find voltage, current, resistance or power from any two known values. This free Ohm's Law calculator works out V = I × R and the power formulas as you type, and shows each step so students can follow the maths.
Enter your values
Fill in any two of voltage, current, resistance or power. The other two are worked out instantly.
Result
V = I × RP = V × I = I²R = V²/RCoro's tip: Power tells you how hot a part gets. A common ¼ W resistor should stay under about 0.125 W for a cool, long life.
Ohm's Law calculator: common questions
What is Ohm's Law?
Ohm's Law says the voltage across a resistor equals the current through it multiplied by its resistance: V = I × R. Rearranged, I = V ÷ R and R = V ÷ I.
How do I calculate power from voltage and current?
Multiply them: P = V × I. If you know the resistance instead, use P = I² × R or P = V² ÷ R.
What wattage resistor do I need?
Choose a resistor rated at least twice the power it will dissipate. A common ¼ W (0.25 W) resistor is comfortable up to about 0.125 W.
For learning and low-voltage hobby circuits (12 V and below). Always check the part's datasheet. Never use these tools to plan mains (230 V) wiring.