LED Resistor Value Calculator

An LED's current rises sharply once the supply exceeds its forward voltage, and without a resistor the current is limited only by the LED's tiny internal resistance. Even a small over-voltage will push current past the rated maximum and burn out the junction. A series resistor sets a fixed current by absorbing the difference between the supply voltage and the LED's forward voltage.

E12 and E24 are standardized geometric sequences of preferred resistor values that manufacturers actually stock. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) with ±10% tolerance, and E24 doubles that to 24 values per decade with ±5% tolerance. You round your calculated value to the nearest one in the series so you can buy the part.

The resistor turns the voltage drop across itself into heat at a rate of P = I² × R. A 470 Ω resistor at 20 mA dissipates only 0.19 W and is fine on a 1/4 W part, but the same resistor at 100 mA dissipates 4.7 W and will scorch a 1/4 W carbon film. Sizing the resistor's wattage rating with a 2× safety margin keeps it cool and reliable.

LEDs of the same nominal color have small variations in forward voltage from the manufacturing process. Wired in parallel through one resistor, the LED with the lowest Vf hogs most of the current while the others stay dim. As the brightest LED heats up, its Vf drops further, the imbalance gets worse, and that LED can fail prematurely. Each parallel LED branch needs its own series resistor.