LED Resistor Calculator

Find the resistor an LED needs from the supply voltage, the LED's forward voltage and the current, plus the nearest standard value and wattage.

LED Color (Typical Forward Voltage)
From the LED datasheet if you have it.
Many indicator LEDs are rated 20 mA; running at 10 mA or less is often plenty bright.
Fractions and decimals both work.
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Ohms
Calculated
Next standard value up
Actual current
Resistor power
Power in LEDs
Efficiency
Resistor Rating

Saved LED Circuits

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NameSupplyLEDsVfCurrentResistorRemove

How to Size an LED Resistor

An LED needs a resistor in series to limit its current. The resistor drops whatever voltage the LED doesn't use: subtract the LED's forward voltage (times the number of LEDs in series) from the supply, and divide by the current you want.

R = (supply voltage − forward voltage × LEDs in series) ÷ current
Resistor power = current² × R

Current is in amps in the formula: 20 mA is 0.020 A. Then pick the next standard resistor value above the result, so the current comes out a little lower rather than higher.

Worked Example

A red LED (2.0 V) on a 5 V supply at 20 mA needs (5 − 2) ÷ 0.020 = 150 Ω. The resistor dissipates 0.020² × 150 = 0.06 W, so a common ¼ W resistor is plenty.

Typical Forward Voltages

LEDTypical VfResistor at 5 V, 20 mAAt 12 V, 20 mA

Forward voltage varies by maker and current; use the datasheet value when you can.

Tips

Wire several LEDs in parallel with one resistor each, not one resistor for all of them, since small differences in forward voltage make them share current unevenly. Pick a resistor rated for at least twice the power it dissipates. High-power LEDs over about 350 mA are better driven with a constant-current driver than a resistor.