LED Resistor Calculator

Work out the series resistor an LED needs, the power it will dissipate, and the nearest value you can actually buy. Driving an LED without one is the fastest way to destroy it.

Supply voltage (V)

LED forward voltage (V)

LED current (mA)

LEDs in series

Series resistor --

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The formula

R = (Vsupply - Vled x n) / Iled

P = (Vsupply - Vled x n) x Iled

Iled in amps: 20 mA = 0.02 A

How it works

An LED is a diode, not a resistor. Above its forward voltage the current climbs almost vertically, so a small rise in supply becomes a large rise in current and the part fails. The resistor is what makes the current predictable.

Typical forward voltages: red and yellow around 1.8 to 2.1 V, green about 2.2 V, blue and white 3.0 to 3.4 V. Check the datasheet where you can — colour is a guide, not a spec.

In series, forward voltages add. Three white LEDs need over 9 V before any current flows at all, which is why the calculator refuses the combination rather than returning a negative resistor.

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