About the LED Resistor Calculator
Series resistor value and wattage for an LED from supply voltage, forward voltage and current.
Also known as: led series resistor, current limiting resistor, led dropping resistor, resistor for led.
How to use this calculator
- Enter supply voltage, led colour, forward current, leds in series and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
R = (V_supply − V_forward) / I_forward
Worked example
Using the values pre-filled in the calculator above:
- Supply voltage = 5 V
- LED colour: Red (2.0 V)
- Forward current = 20 mA
- LEDs in series = 1
the calculator returns:
- Resistor value: 150 Ω
- Nearest standard (E24): 150 Ω
- Power dissipated: 60 mW
- Recommended rating: 1/8 W is fine
Tips & common mistakes
- Round up to the next standard resistor — a slightly dimmer LED lasts longer.
- For LEDs in parallel give each one its own resistor.