LED Resistor Calculator
Find the right current-limiting resistor for an LED from supply voltage, forward voltage and current with this free calculator.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the supply voltage, LED voltage and current.
- Press Calculate.
How it works
Resistor = (supply voltage − LED voltage) ÷ LED current. It drops the extra voltage so the LED gets the right current.
Worked example
12 V, 2 V LED, 20 mA. = 500 Ω.
Common mistakes
- Ignoring the LED forward voltage — subtract it from the supply before dividing by current.
- Picking a resistor with too low a power rating, which then overheats.
Frequently asked questions
How do I size an LED resistor?
R = (supply voltage − LED forward voltage) ÷ LED current. For 12 V, a 2 V red LED at 20 mA: (12 − 2) ÷ 0.02 = 500 Ω, so use 510 Ω.
What power rating does the resistor need?
Power = current² × resistance; choose the next standard rating above it (0.25 W is usually enough for small indicator LEDs).
What if I don't know the LED's forward voltage?
Typical values are ~1.8–2.2 V (red), ~3.0–3.4 V (blue/white). Check the LED's datasheet for an accurate figure.
Related tools
- Ohms Law Calculator
- Voltage Divider Calculator
- Parallel Resistance Calculator
- Series Resistance Calculator
- Capacitor Energy Calculator
- Inductive Reactance Calculator
Explore more in Electrical, Electronics, Solar & Energy.
Cite this tool
Zerdly. (2026). LED Resistor Calculator. Retrieved from https://www.zerdly.com/tools/led-resistor-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.