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LED Resistor Calculator

Find the series resistor for one or more LEDs, with the nearest standard value and power rating.

Result

Enter your values and select Calculate to see the result.

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How the LED Resistor Calculator works

LEDs need a resistor in series to limit the current. Enter the supply voltage, the LED's forward voltage and the current you want (often 20 mA), and how many LEDs are in series. The calculator finds the exact resistance, the next standard E12 value up, the current that value gives and the power the resistor must handle.

Formulas

Resistor

R = (Vsupply − n × Vf) ÷ I

Resistor power

P = (Vsupply − n × Vf) × I

Examples

A red LED on 5 V

(5 − 2) ÷ 0.02 = 150 Ω, a standard value; the resistor dissipates 0.06 W.

A white LED on 12 V

(12 − 3.2) ÷ 0.02 = 440 Ω; the next E12 value is 470 Ω, giving about 18.72 mA.

Frequently asked questions

Why round up, not down?

A larger resistor gives slightly less current, which is safe; a smaller one could overdrive the LED.

Can LEDs share one resistor in parallel?

It's better to give each parallel LED its own resistor — small differences make one LED hog the current.

What resistor wattage do I need?

At least the calculated power, ideally double it. ¼ W resistors suit most single-LED circuits.

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