How this calculator works
An LED isn't a resistor — past its forward voltage, current rises steeply for almost no extra voltage, so an LED wired straight to a supply will self-destruct. A series resistor absorbs the leftover voltage and turns it into a fixed current the LED can survive.
- V_supply — supply voltage (V)
- V_forward — LED forward voltage per unit (V)
- n — number of LEDs in series
- I_forward — target forward current (A)
- R — resistor value (Ω)
More detail
Why the power rating is doubled
The wattage figure above is the resistor's actual dissipation at your target current — but resistors run right at their rated wattage get hot, drift in value, and fail early. Doubling the calculated power (using at least a 2× margin, e.g. a 1/4 W resistor for a calculated 0.1 W load) keeps the resistor running cool and its value stable over the LED's lifetime, at the cost of a slightly bigger part.
Typical forward voltages. Red and yellow LEDs sit around 1.8–2.2 V, green and blue around 3.0–3.4 V, and white LEDs are usually similar to blue (~3.0–3.4 V) since they're a blue die with phosphor. When wiring multiple LEDs in series, use the sum of each one's forward voltage as V_forward × n.
Frequently asked questions
What resistor do I need for a red LED on a 9V battery?
With a typical red LED at 2.0 V forward voltage and a 20 mA target current: R = (9 − 2.0) / 0.02 = 350 Ω. The nearest E12 stock value is 330 Ω, which runs slightly above 20 mA — fine for most indicator LEDs.
Can I run three blue LEDs in series from 12V?
Three blue LEDs at 3.2 V each need 9.6 V total, leaving 2.4 V of headroom from a 12 V supply. At 20 mA: R = (12 − 9.6) / 0.02 = 120 Ω, an exact E12 value. If your supply voltage barely clears the total forward voltage, the resistor value gets very small and current becomes sensitive to supply fluctuation — leave more headroom if you can.
Why does the calculator say the LED won't light?
If supply voltage doesn't exceed the total forward voltage (V_forward × count), no series resistor can make current flow — the LED needs at least a few hundred millivolts of headroom above its forward voltage to turn on at all. Lower the LED count or raise the supply voltage.