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LED resistor calculator

The series resistor for one LED or a string.

Try the circuit in the labBuild it on the simulator's bench and measure it.

How it is worked out

R = (Vsupply − n × VF) ÷ I. The resistor drops the voltage the LEDs do not use and sets the current. Round up to the next standard value so the current comes out a little lower, never higher.

Example: a white LED (3.0 V) from 5 V at 15 mA needs (5 − 3) ÷ 0.015 = 133 Ω; the next E12 value is 150 Ω, giving 13 mA and 27 mW in the resistor.

Most 5 mm LEDs are rated 20 mA continuous; indicator LEDs look bright at 2 to 10 mA. Never connect an LED straight across a supply: with no resistor the current is limited only by the supply and the LED burns out.

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