Ohm's law calculator

The formula

Ohm's law links voltage (V), current (I) and resistance (R): V = I × R, so I = V ÷ R and R = V ÷ I. The power is P = V × I. Enter any two values and the calculator works out the third and the power.

You can type values the way they're printed on parts and schematics: 4.7k is 4,700 Ω, 2M2 is 2.2 MΩ, 20m is 0.02 A (20 mA), 100u is 0.0001.

Worked example

An LED needs about 20 mA and drops 2 V, fed from 12 V. The resistor has the remaining 10 V across it, so R = 10 V ÷ 0.02 A = 500 Ω - use the next standard value up, 510 Ω or 560 Ω. It dissipates 10 V × 0.02 A = 0.2 W, so a 1/4 W resistor is enough, a 1/2 W one runs cooler.

In repair work

Ohm's law is how you sanity-check a circuit with a meter: measure the voltage across a resistor of known value and you know the current through it without breaking the circuit. A low-value resistor that's cooked usually means the current through it was far higher than designed - look for the shorted part downstream. See where parts like this sit on real boards in our board library.

Questions

How do I enter milliamps?

Type 20m for 20 mA (0.02 A), or 0.02. The same prefixes work everywhere: k (kilo), M (mega), m (milli), u or µ (micro), n (nano), p (pico).

Why does it show the power too?

Because it's the next thing you need: a resistor or a transistor has to be rated for the power it dissipates, ideally with a good margin.