Ohm's Law Calculator

Ohm's law relates the voltage across a resistor to the current through it and its resistance:

V = I * R

with V in volts, I in amperes and R in ohms. Rearranged: I = V / R and R = V / I. The power turned into heat follows from any two of them:

• P = V * I
• P = I² * R
• P = V² / R

Enter any two of voltage, current, resistance and power, and the calculator finds the other two.

Examples

1. Current: 12 V across 6 Ω.

   I = 12 / 6 = 2 A, using P = 12 * 2 = 24 W

2. Resistance: a lamp drawing 0.5 A at 230 V.

   R = 230 / 0.5 = 460 Ω, using 115 W

3. A resistor's power: 220 Ω carrying 20 mA (0.02 A).

   P = 0.02² * 220 = 0.088 W, so a 0.25 W resistor is enough

Each field has its own unit menu, so values can be entered in mA, kΩ or kW directly.

Frequently asked questions

Does Ohm's law always hold?

For resistors and metal wires at a steady temperature, yes. Diodes, LEDs and transistors don't follow it: their current isn't proportional to the voltage. In AC circuits with capacitors or coils, the impedance (Z) takes the place of the resistance.

How do resistors add up?

In series, they add: R = R1 + R2. In parallel, their inverses add: 1 / R = 1 / R1 + 1 / R2, so two 100 Ω resistors in parallel make 50 Ω.