How do I calculate a resistor for an LED?

Subtract the total LED forward voltage from the supply voltage, then divide the remaining voltage by the desired current in amperes.

Why does the calculator round an E12 or E24 resistor upward?

Choosing the first standard value at or above the exact resistance keeps the calculated current at or below the requested target.

Are LED forward-voltage presets exact?

No. They are editable starting points. Forward voltage depends on the exact LED, current, and temperature, so the component datasheet should be used for final designs.

LED Resistor Calculator

Calculate a current-limiting resistor, a practical power rating, and safe E12 or E24 alternatives for a series LED circuit.

LED Resistor Calculator

Calculate a current-limiting resistor, a practical power rating, and safe E12 or E24 alternatives for a series LED circuit.

Live circuit preview

Series LED circuit

Recommended series resistor

150 ΩExact value

3 V across the resistor at 20 mA

Voltage drop: 3 V
Resistor power: 60 mW
Use at least: 1/8 W
Actual current: 20 mA

Formula guide

How the resistor is calculated

1. LED voltage

V_LED = LED count × forward voltage

2. Resistor value

R = (V_S − V_LED) ÷ I

3. Power

P = (V_S − V_LED) × I

The wattage recommendation applies a 2× margin by default and selects the next common resistor rating. LED forward voltage varies with the exact part, current, and temperature; use the device datasheet for a final design.