LED resistor calculator

Why LEDs need a resistor

An LED drops a nearly fixed forward voltage and will draw as much current as the supply allows, quickly burning out. A resistor in series sets the current: it takes up the difference between the supply and the LED voltage. By Ohm's law, R = (Vsupply − Vf) ÷ I.

Worked examples

• A red LED (2 V) on 5 V at 20 mA: (5 − 2) ÷ 0.020 = 150 Ω, a standard value. The resistor dissipates 60 mW, so a 1/8 W part is fine.
• Three white LEDs (3.2 V each) in series on 12 V: (12 − 9.6) ÷ 0.020 = 120 Ω.
• A red LED on 9 V: 350 Ω is not a standard value, so use 360 Ω, giving 19.4 mA. It dissipates 136 mW: use a 1/2 W resistor, or at least 1/4 W.

Typical forward voltages

Check your LED's datasheet. 20 mA is a common maximum for small 3 mm and 5 mm LEDs; 5–10 mA is often bright enough for indicators.

The page subtracts the forward voltage of all LEDs in series from the supply and divides by the current to get the exact resistance. It rounds up to the next value in the E24 series, the standard resistor values, so the current never exceeds your target, then recalculates the actual current and the power in the resistor. The rating suggestion doubles the power for a safety margin.