LED Resistor Calculator
Calculate the resistor value needed for an LED circuit. Enter supply voltage, LED forward voltage, and desired current.
Red: 1.8–2.2V · Green: 2.0–3.5V · Blue/White: 3.0–3.5V
Standard: 20 mA
Enter values to calculate the resistor.
LED Resistor Formula
R = (Vsupply − Vforward) / ILED
The resistor limits current through the LED to prevent damage. Without it, the LED would draw too much current and burn out.
Example: 5V supply, red LED (2V forward), 20 mA desired current.
R = (5 − 2) / 0.020 = 3 / 0.020 = 150 Ω
Power = 0.020² × 150 = 0.06 W → use a 1/4 W resistor
R = (5 − 2) / 0.020 = 3 / 0.020 = 150 Ω
Power = 0.020² × 150 = 0.06 W → use a 1/4 W resistor
Standard Resistor Values (E12 Series)
Standard resistors come in fixed values: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 (and multiples). Choose the nearest standard value equal to or greater than your calculated value.
FAQ
What happens if I use a larger resistor?
The LED will be dimmer. A smaller resistor means more current and a brighter LED, but exceeding the LED's rated current will shorten its life.
Last updated: 2026-08-08