Electronics Simplified

Ohm's Law Calculator

Any two of volts, amps, ohms or watts give you the rest.

Type any two. The last two you typed are used; the others are worked out.

Voltage
12 V
Current
200 mA
Resistance
60 Ω
Power
2.4 W

The formula

V = I × R P = V × I

Cover the one you want in the V-I-R triangle: V = I × R, I = V ÷ R, R = V ÷ I. Power in watts is P = V × I (also I² × R and V² ÷ R). Use volts, amps and ohms: 20 mA = 0.02 A, 4.7 kΩ = 4,700 Ω.

Worked example: How much current does a 12 V, 60 Ω heater element draw?

  1. I = V ÷ R = 12 ÷ 60 = 0.2 A (200 mA).
  2. P = V × I = 12 × 0.2 = 2.4 W of heat.

Common questions

Does Ohm's law work for everything?

It works for resistors and wires. LEDs, diodes and transistors don't follow it on their own (their resistance changes with voltage), which is why an LED needs a resistor to set its current.

Why does my answer come out 1,000 times too big or small?

Usually a milli or kilo mix-up. Convert to plain volts, amps and ohms first: mA ÷ 1000, kΩ × 1000.

Everything on this subject: Ohm's Law and Circuit Basics, Simplified