LED Resistor Calculator
Calculate the series resistor value and power rating needed to safely drive an LED.
Estimates for planning only — verify against your LED datasheet and applicable safety standards before wiring.
How it works
- 1Enter your supply voltage, the LED’s forward voltage from its datasheet, and the operating current in mA.
- 2The calculator applies R = (Vsupply − Vforward) ÷ I to find the current-limiting resistance.
- 3Read the resistor power and the recommended wattage, then pick the next standard resistor value up.
Use cases
- Driving indicator LEDs from 3.3 V or 5 V on Arduino or Raspberry Pi.
- Designing panel LED indicators from 12 V or 24 V DC supplies.
- Choosing replacement resistors when repairing LED strips or boards.
Frequently asked questions
How is the series resistor value found?
R = (Vsupply − Vforward) ÷ I, where I is in amperes. For 5 V supply, a 2 V LED at 20 mA: (5 − 2) ÷ 0.02 = 150 Ω.
Should I use the exact calculated value?
Round up to the next standard resistor (E12/E24 series). A slightly higher resistance dims the LED marginally but protects it from overcurrent.
What resistor wattage do I need?
Power is P = (Vsupply − Vforward) × I. The 5 V/2 V/20 mA example dissipates 60 mW — a ¼ W resistor is plenty. Choose at least twice the calculated power.
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