Ohm’s Law Calculator
Solve for voltage, current, or resistance — and get the power, too.
Power: 40 W
Ohm’s law is V = I × R, rearranged to R = V ÷ I. A 12 V battery driving 3 A through a resistor means R = 12 ÷ 3 = 4 Ω, and the power dissipated is P = V × I = 12 × 3 = 36 W.
How Ohm’s law works
In a resistive circuit, the voltage across a component, the current through it, and its resistance are locked together by one relationship: V = I × R. Push harder (more voltage) and more current flows; add resistance and the current drops. Rearrange the same equation to solve for whichever quantity you are missing, or reach for the scientific calculator when the prefixes get messy.
V = voltage (V), I = current (A), R = resistance (Ω), P = power (W)
Worked example
A 12 V battery drives 3 A through a resistor. Find its resistance and the power it dissipates:
- 1 Rearrange for resistance. From V = I × R, divide both sides by I: R = V ÷ I.
- 2 Substitute the values. R = 12 V ÷ 3 A = 4 Ω.
- 3 Find the power dissipated. P = V × I = 12 × 3 = 36 W (equivalently I²R = 3² × 4 = 36 W).
Ohm’s law rearranged
Three forms of one equation, plus the matching power expressions.
| Solve for | Formula | Units |
|---|---|---|
| Voltage | V = I × R | volts (V) |
| Current | I = V ÷ R | amperes (A) |
| Resistance | R = V ÷ I | ohms (Ω) |
| Power | P = V × I = I²R = V²/R | watts (W) |
Units, scope, and common mistakes
Keep your prefixes straight. Convert to base units before calculating: 1 kΩ = 1000 Ω, 1 mA = 0.001 A. Mixing milliamps with kilohms is the single most common error here.
This is for DC, resistive circuits. Ohm’s law in this form assumes a constant resistance. Diodes, transistors, and other non-ohmic components do not obey V = IR, and AC circuits with capacitors or inductors need impedance instead.
Power has three faces. P = VI, P = I²R, and P = V²/R all give the same answer — pick the one that uses the values you already have.