Find the series resistor an LED needs. Enter the supply voltage, the LED forward voltage and the current you want. You get the exact value, the nearest standard E12 value and its power rating. Free, private, and runs entirely in your browser.
Ohm's law gives the resistor: subtract the LED forward voltage from the supply voltage and divide by the current you want. The result is rounded up to the next standard E12 value so that the current stays within the limit.
What is a typical LED forward voltage? About 1.8 to 2.2 V for red, 3 to 3.4 V for white and blue, depending on the LED.
Why round up the resistor value? A slightly larger resistor gives a little less current, which is safer for the LED than a smaller one.
A red LED with a forward voltage of 2 V and a current of 20 mA runs on a 12 V supply. R = (12 − 2) ÷ 0.020 = 500 Ω. The nearest E12 value at or above that is 560 Ω, which gives about 17.9 mA, and the resistor dissipates about 179 mW, so a 1/2 W part is suitable.
Can I connect LEDs in parallel? It is better to give each LED its own resistor, since small differences in forward voltage would otherwise cause uneven current.
What current should I choose? Often 10 to 20 mA for standard indicator LEDs. Check the datasheet.
The first practical visible-spectrum light-emitting diode was demonstrated by Nick Holonyak Jr. at General Electric in 1962. LEDs behave like diodes, so they need a current-limiting resistor, and the E12 series of preferred values is used so that standard parts are available at every decade.
Use this calculator when wiring an indicator LED on a breadboard, in a car or on a microcontroller. For a 5 V supply, a 2 V LED and 10 mA, the resistor is 300 Ω, and the nearest E12 value is 330 Ω. Always check the datasheet.
What is the E12 series? A set of 12 standard resistor values per decade, such as 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82.
Can I use a higher current? Only up to the LED rating, and brighter output also means more heat.