LED Resistor Calculator - Find Series Resistor for LEDs

Use this LED resistor calculator to find the correct series resistor for one or more LEDs. Enter your supply voltage, LED forward voltage, desired LED current and how many LEDs are in series. The calculator will output the resistor value and minimum power rating.

How the LED resistor calculator works

LEDs need a series resistor to limit current and prevent damage. This calculator uses the classic formula:

R = (V_supply − N × V_f) / I

V_supply – your power supply voltage (for example 5 V, 9 V, 12 V)
V_f – LED forward voltage (around 2.0 V for red, 3.0–3.4 V for white/blue)
I – desired LED current in amps (for example 20 mA = 0.02 A)
N – number of LEDs in series

The power dissipated in the resistor is calculated as P = V_R × I, where V_R is the voltage drop across the resistor. In practice, you should choose a resistor with a power rating at least 2× the calculated value for safety.

Typical example: 12 V supply, one red LED (2.0 V), 20 mA. R ≈ (12 − 2) / 0.02 = 500 Ω, so you would choose the nearest standard value (e.g. 470 Ω or 510 Ω) and an appropriate power rating.