LED Resistor Calculator
Find the correct current-limiting resistor for any LED by entering your supply voltage, LED forward voltage, and desired current.
Resistor Value
—Ω
How to Use This Calculator
Enter your supply voltage (e.g. 5V for Arduino, 12V for a car circuit), your LED's forward voltage (found on the datasheet — typically 1.8V–3.5V), and your desired current (20 mA is standard for most through-hole LEDs). Click Calculate Resistor to get the result.
Formula
The LED resistor formula is derived from Ohm's Law, subtracting the voltage drop across the LED from the supply:
R = (Vs − Vf) / I
Where Vs = supply voltage, Vf = LED forward voltage, and I = current in amps.
Example
Scenario: 5V Arduino powering a red LED (Vf = 2.0V) at 20 mA
R = (5 − 2.0) / 0.020
R = 3.0 / 0.020
R = 150 Ω → use a 150 Ω or 180 Ω resistor
R = (5 − 2.0) / 0.020
R = 3.0 / 0.020
R = 150 Ω → use a 150 Ω or 180 Ω resistor