LED Series Resistor Calculator

Subtract the LED forward voltage from the supply voltage, then divide by the current in amps: R = (Vs - Vf) / I. A 20 mA current is 0.02 A, so 3 V across the resistor gives 150 ohm.

It depends on colour. Red is around 1.8 to 2.2 V, yellow and green a little higher, while blue and white are typically 3.0 to 3.4 V. Use the exact value from your LED's datasheet for the best result.

E12 is a standard series of resistor values with 12 steps per decade at 10 percent tolerance. The tool picks the closest stocked value on a log scale, so an exact 500 ohm result maps to 470 ohm.

From P = I^2 x R using the current in amps and the exact resistance, which equals the voltage across the resistor times the current. At 20 mA and 150 ohm that is about 0.06 W.

The resistor turns the excess voltage into heat, so pick a part rated comfortably above the power shown. A common 0.25 W resistor is plenty for a single small indicator LED.

If the supply is at or below the LED forward voltage there is no voltage left for the resistor, so no positive value can set the current. The tool rejects that input rather than return a meaningless number.

For LEDs in series, add their forward voltages and enter the total as Vf, keeping the same current. For separate branches, size a resistor for each branch, since paralleling one resistor across several LEDs is unreliable.

The tool reports the nearest E12 value, but choosing the next value up is safer because it lowers the current slightly and protects the LED. A value down raises current and brightness but adds heat.

It runs entirely in your browser. The voltages and current you enter are never uploaded or stored, so the calculation stays completely local.