LED Series Resistor Calculator

How it is calculated

The tool uses Ohm's law on the voltage left over after the LED: R = (Vs − Vf) ÷ I, with Vs the supply voltage in volts, Vf the LED forward voltage in volts and I the target current. The current is typed in milliamps and converted to amps (÷ 1000). It also gives the resistor's power P = (Vs − Vf) × I and the LED's power Vf × I, both in watts. The supply must be higher than the forward voltage and the current must be above zero, otherwise there is no result. The ohm value is exact, not rounded to a standard resistor series, so pick the next higher standard value.

Common mistakes

• Entering the current in amps. The field is in milliamps, so 0.02 would be read as 0.02 mA and give a 150,000 Ω resistor.
• Using one forward voltage for every colour. Red is near 2 V while blue and white are about 3 V or more, so a 3.3 V supply cannot drive a 3.3 V LED through a resistor and the tool gives no result.
• Rounding the resistor down to the nearest standard value. That raises the current above the target, so round up and accept slightly less brightness.
• Forgetting the supply may sag or run high. A battery that reads 4.2 V when full changes the current, so use the highest supply voltage the circuit will see.
• Ignoring resistor wattage. The result is small here, but a 12 V supply with a 2 V LED at 20 mA dissipates 0.2 W, which is too much for a tiny 0.125 W resistor.