LED Resistor Calculator

Calculates the series resistor and its power for an LED from the supply voltage, the LED forward voltage (Vf) and the current. Use it when you power LEDs from an Arduino, USB or a battery.

How to use

1. Enter the "Supply voltage (Vs)" and the "LED current" in mA.
2. Choose the "LED colour" to fill in a typical Vf, or type the datasheet value in "LED forward voltage (Vf)".
3. Set "LEDs in series" and the "Standard resistor series" (E12 or E24) to see the "Resistor to use", the actual current and the "Suggested resistor rating".
4. For many LEDs, enter the "Number of LEDs" under "Many LEDs" to see the series strings and their resistors.

How it works

• R = (Vs − number in series × Vf) ÷ current. The next standard value up is chosen so the actual current stays at or below the target.
• The suggested rating is the smallest of 1/8, 1/4, 1/2, 1, 2 and 3 W that is at least twice the power in the resistor.
• The tool is for low-voltage DC up to 60 V (batteries, USB, adapters). It does not cover LEDs connected directly to mains power.

Questions and answers

What resistor do I need for an LED on 5V?

A red LED (Vf 2 V) at 20 mA needs (5 − 2) ÷ 0.02 = 150 Ω. A white LED (Vf 3.2 V) gives 90 Ω, so the next E24 value up, 91 Ω, is used.

How do I calculate the LED resistor for 12V?

Series LEDs waste less power in the resistor. Three white LEDs at 20 mA need (12 − 3 × 3.2) ÷ 0.02 = 120 Ω.

What wattage should the resistor have?

The resistor power is the voltage across it × the current. For 5 V, a red LED and 20 mA, that is 3 V × 0.02 A = 0.06 W, so with a 2× margin a 1/8 W resistor is enough.

Can several LEDs in parallel share one resistor?

It is not recommended. The current crowds into the LED with the lowest Vf, which gets hot and can fail first. Give each parallel string its own resistor.