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LED Resistor Calculator
Find the series resistor value for an LED.
Resistor
—
Series resistor.
| Power in resistor | — |
|---|---|
| Nearest E12 | — |
| Voltage across R | — |
| — |
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How the LED Resistor Calculator Works
An LED needs a series resistor to limit its current, or it burns out. The resistor drops the difference between the supply voltage and the LED’s forward voltage, at the desired current. Ohm’s law gives the value: R = (Vsupply − Vforward) ÷ current. Powering a 2 V LED at 20 mA from 12 V needs a 500 Ω resistor.
Also check the resistor’s power rating — the calculator shows the milliwatts it dissipates — and pick a standard (E12) value at or above the calculated resistance.
R = (supply voltage − LED forward voltage) ÷ current
Frequently Asked Questions
What resistor do I need for an LED?
R = (supply − LED forward voltage) ÷ current. A 2 V LED at 20 mA on 12 V needs about 500 Ω.
Why does an LED need a resistor?
LEDs draw runaway current without one and quickly burn out. The resistor limits the current to a safe level.
What power rating should the resistor have?
Above the wattage it dissipates — shown here in milliwatts. A quarter-watt resistor covers most small LEDs.
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