Current Limiting Resistor Calculator

About this calculator

A current-limiting resistor is placed in series with a component like an LED to keep the current at a safe, specified level, since LEDs and many other loads have a low, non-linear resistance that would otherwise let current spike and destroy them. The resistor value is found with a rearranged Ohm's law: R = (supply voltage − load's forward voltage) ÷ target current, and its power rating is found with P = I² × R (or I × V across the resistor), rounded up to the nearest standard resistor wattage to leave a safety margin.

This calculation is one of the first things taught in basic electronics, and it's used constantly by hobbyists wiring up LEDs on breadboards, Arduino and Raspberry Pi project builders, and electronics technicians repairing or prototyping circuits, since picking too small a resistor burns out the LED while picking too large a one leaves it dim.

This calculator takes your supply voltage, the load's forward voltage, and your target current, then returns both the required resistor value and the minimum power rating it needs to survive without overheating.