Ohm's law.

Enter any two of voltage, current, resistance, or power. The other two auto-calculate.

// MNEMONIC

"Eagles fly over rabbits and Indians."

The Eagle (E) flies over the Rabbits (R) and Indians (I) — that's how they sit in the circles. Cover the value you want:

Cover E → I × R (E = I × R)
Cover I → E / R (I = E / R)
Cover R → E / I (R = E / I)

Ohm's Law: E = I × R

Watt's Law: P = E × I

// CALCULATOR · INPUT ANY TWO FILL ANY TWO FIELDS

// E — Voltage

// I — Current

// R — Resistance

// P — Power

// All 12

Formulas worth memorizing

E = I × R

E = P / I

E = √(P × R)

I = E / R

I = P / E

I = √(P / R)

R = E / I

R = E² / P

R = P / I²

P = E × I

P = E² / R

P = I² × R

// How it works

Ohm's law, plus the power check

Ohm's law says the voltage across a resistance equals the current through it times the resistance. Add the power law and there are four quantities (voltage E, current I, resistance R, power P), and any two give you the other two.

E = I × R, I = E / R, R = E / I, and P = E × I = I² × R = E² / R.

Example. 12 V across a 220 Ω resistor: I = 12 / 220 = 0.0545 A (54.5 mA) and P = 12² / 220 = 0.655 W. The calculator fills the inputs with plain numbers and prints the same result underneath with prefixes: I = 54.55 mA, P = 654.5 mW.

Gotcha: check the power, not just the value. If that 220 Ω is an ordinary ¼ W resistor it is dissipating 2.6× its rating and will cook. Pick a part rated for about twice the calculated power, a 1 W resistor here. And watch units: this calculator works in volts, amps, ohms and watts, so enter 20 mA as 0.02 and 4.7 kΩ as 4700.

// Common questions

Questions people actually ask

What does E stand for in Ohm's law?

Electromotive force, the older name for voltage. E and V mean the same thing: E = I × R and V = I × R are the same equation.

How do I enter milliamps or kilohms?

Convert to base units first: 20 mA is 0.02 A, 4.7 kΩ is 4700 Ω and 250 mW is 0.25 W. Most answers that are off by a factor of 1,000 come from a missed conversion.

Does Ohm's law work for LEDs?

Not for the LED itself, because an LED's current is not proportional to its voltage. Apply Ohm's law to the series resistor instead: the voltage across it is the supply minus the LED's forward voltage. The LED resistor calculator does exactly that.

Why does the calculator only take two values?

Voltage, current, resistance and power are linked by two equations, so any two of them fix the other two. If you type a third value the calculator asks you to clear one, because the three might contradict each other.

How do I work out how much power a resistor dissipates?

Use P = I² × R or P = E² / R. For example, 9 V across 470 Ω gives I = 9 / 470 = 19.1 mA and P = 81 / 470 = 0.172 W, which is fine for a ¼ W resistor.