LED Series Resistor Calculator

Work out the current-limiting resistor an LED (or a row of LEDs) needs, plus the nearest standard value, its wattage, and its color bands.

Supply voltage

LEDs in series

LED color (sets typical Vf)

LED current (If) — 20 mA is a safe default for a standard 5 mm LED

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How this is worked out

R = (V_supply − N × V_f) ÷ I_f — the resistor soaks up whatever voltage is left over after N LEDs in series, at the current you want flowing through them.

The result is rounded up to the nearest standard (E12) resistor value, which very slightly lowers the LED current — safer than rounding down. Wattage is the calculated power (I²×R) with a ×2 safety margin, then rounded up to a standard resistor wattage.

Typical forward voltages: Red/Orange/Yellow ≈ 1.8–2.1 V, Green ≈ 2.0–2.2 V, Blue/White ≈ 3.0–3.4 V. Check your LED's datasheet if you have one — it overrides these defaults.