LED Series Resistor Calculator
Run your LEDs at the right current the simple way. Enter the supply voltage, the LED forward voltage and current, and how many are in series, and get the resistor value, the nearest standard part, and the power rating it needs.
Typical forward voltage: red ~1.8–2.2 V, green ~2.0–3.0 V, blue/white ~3.0–3.4 V. Common current 20 mA. Check the datasheet.
Formula & how it works
The resistor drops whatever the LEDs do not: R = (Vsupply − n × Vf) ÷ I, with I in amps. Its power is (Vsupply − n × Vf) × I. The nearest standard uses the E12 series, rounded up so current stays at or below target.
Worked example
A 12 V supply, one LED with a 2 V forward drop at 20 mA: R = (12 − 2) ÷ 0.02 = 500 Ω. The nearest standard is 560 Ω. The resistor dissipates 10 × 0.02 = 0.2 W, so a half-watt part gives comfortable margin.
Driving LEDs safely
Why LEDs need a resistor
An LED is not a resistor — its current rises steeply once past the forward voltage, so connecting one straight across a supply lets current run away and burns it out. A series resistor absorbs the extra voltage and sets a steady current, which is what a healthy, consistent brightness depends on.
Series stacking
Putting LEDs in series adds their forward voltages, so fewer volts are left for the resistor and the resistor value drops. There must be enough supply headroom for all of them plus a little for the resistor; if the total forward voltage approaches the supply, add fewer LEDs or raise the supply.
Round up, then check power
Standard resistor values rarely land exactly on the calculated number. Rounding up nudges current slightly below target, which is the safe direction for the LED. Then glance at the power figure — most indicator LEDs need only a small resistor, but higher currents or bigger voltage drops can call for a half-watt or larger part.