This LED resistor calculator sizes the series resistor that keeps an LED within its safe current. Enter the supply voltage, LED forward voltage, and desired current above.
Formula (Ohm’s Law)
R = (Vsupply − VLED) ÷ ILED Presistor = (Vsupply − VLED) × ILED
Example
12 V supply, 2 V LED, 20 mA: R = (12 − 2) ÷ 0.02 = 500 Ω; power = 10 × 0.02 = 0.2 W, so use a 0.5 W resistor.
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Frequently Asked Questions
How do I calculate an LED resistor?
Subtract the LED forward voltage from the supply voltage, then divide by the LED current. For 12 V, a 2 V LED at 20 mA needs 500 ohms.
What resistor do I need for a 3V LED on 12V?
Divide 12 minus 3 by your current; at 20 mA that is 9 divided by 0.02, or 450 ohms.
What wattage resistor should I use?
Multiply the voltage across the resistor by the current. Choose a resistor rated at least twice that power for safety.