LED Resistor Calculator

LED resistor calculator tells you what resistor you should use when connecting LEDs in series or parallel.

Series resistor495 Ω(12 − 2.1) ÷ 20 mA

Nearest standard E12 value470 Ωround UP if in doubt — a larger resistor means slightly less current and a slightly dimmer but safer LED

Voltage across the resistor9.9 V

Voltage across the LEDs2.1 V

Power in the resistor198 mWchoose a resistor rated at least twice this

Power in the LEDs42 mW

Efficiency of the dropper17.5%the resistor wastes the rest as heat, which is why a constant-current driver is used for anything beyond indicator LEDs

Maximum LEDs in series on this supply4 with headroom for a resistormore LEDs in series drop more of the supply usefully, so the dropper wastes less

Dropper efficiency with 4 LEDs70%

Never parallel LEDs on one resistorcurrent will not shareforward voltages differ slightly between parts, so the lowest-Vf LED takes most of the current and fails first — each parallel branch needs its own resistor

The formula

R = (V_supply − V_f) ÷ I_f

An LED has no useful resistance

A diode's current rises exponentially with voltage, so a fraction of a volt over the forward drop produces a destructive current. LEDs are therefore driven by controlling the current, not the voltage, and the simplest way is a series resistor sized to drop whatever the supply leaves over.

That resistor wastes the difference as heat. Running a 2.1 V LED from 12 V puts 82% of the power in the resistor, which is fine for an indicator and hopeless for lighting. Putting more LEDs in series recovers most of it, which is why LED lamps run long strings at high voltage — and why anything beyond indicators uses a switching constant-current driver.

Never put LEDs in parallel across one resistor. Forward voltages vary by tens of millivolts between nominally identical parts, and because current depends exponentially on that voltage, the LED with the lowest drop takes a disproportionate share, runs hottest, drops further as it warms, and takes more still. Each parallel branch needs its own resistor.