LED Resistor Calculator

Subtract the total LED forward voltage from the supply voltage, then divide by the LED current in amps: R = (Vs minus n × Vf) / If. A blue LED with a 3.0 V forward voltage running at 20 mA from a 5 V supply needs (5 minus 3.0) / 0.02 = 100 Ω. The calculator also reports the nearest standard preferred value and the power the resistor dissipates.

Forward voltage depends on the LED colour and chemistry. Typical values are about 1.8 V for red, 2.0 V for yellow, 2.1 V for green, 3.0 V for blue and 3.2 V for white. These are starting points: the datasheet value at your chosen current is always more accurate, and high-power LEDs often sit higher. The dropdown in the calculator fills in these typical values for you.

LEDs in series can share one resistor because the same current flows through every device: add up the forward voltages and make sure the total stays comfortably below the supply voltage. Avoid putting LEDs in parallel on a single resistor: small differences in forward voltage make one LED hog the current, so it runs bright and hot while the others stay dim. Give each parallel branch its own resistor.

The resistor dissipates P = (Vs minus n × Vf) × If. A green LED at 20 mA on a 24 V panel drops about 21.9 V across the resistor, which is roughly 0.44 W, so a common 0.25 W resistor would overheat. Choose a resistor rated at least double the calculated dissipation: in that example a 1 W part gives a sensible margin.

Round up to the next standard value where you can. A slightly larger resistance reduces the LED current a little, which dims it marginally and extends its life. Rounding down increases the current beyond the design figure and erodes the safety margin against the LED's maximum rating. The calculator suggests a nearby standard value; check whether it sits above or below the exact figure before ordering.