LED Resistor Value Calculator
Description
An LED needs a resistor in series to drop the supply voltage it doesn't use down to a safe current — without one, it draws as much current as the supply can push and burns out almost immediately. This calculator gives you the exact resistance for your supply voltage, your LED's forward voltage, and the current you want it to run at, then snaps that number up to the nearest resistor value actually sold off the shelf (rounding up is the safe direction — it gives slightly less current, not more).
It also reports the power the resistor will dissipate, so you can check it against a resistor's wattage rating before you solder it in — a ¼-watt resistor run well past its rating runs hot and can fail.
A typical small LED's forward voltage is usually somewhere around 1.8-2.2V for red/orange/ yellow, and 2.8-3.4V for green/blue/white, but it varies by LED — check your specific LED's datasheet rather than assuming a number, especially for anything other than a basic 3mm or 5mm indicator LED.
Sources
The resistor formula is Ohm's law applied to an LED circuit — R = (Vsupply − n × VLED) / I — plain electrical physics, not a modeling-specific standard. The standard resistor value this calculator snaps to is the E12 series defined by IEC 60063 (12 preferred values per decade, ±10% tolerance), the most commonly stocked series for hobby electronics.
Disclaimer: always check your specific LED's datasheet for its forward voltage and maximum current rating, and your resistor's wattage rating against the power this calculator reports — don't assume a typical value is correct for your part.