LED Resistor Calculator

An LED's forward voltage barely changes as current through it rises — which sounds convenient and is actually the problem. Without something else in the circuit limiting current, a tiny rise in supply voltage drives a large, uncontrolled rise in current, and an LED destroys itself in milliseconds run this way. The resistor is what actually sets the current: it drops whatever voltage the LED doesn't, and Ohm's law pins the resulting current to a safe, predictable value.

Forward voltage depends on the semiconductor material an LED is made from, which is also what determines its colour — red, yellow and orange LEDs typically drop around 2V, while green, blue and white ones drop more, often 3V or higher. This is why a resistor calculated for a red LED is usually wrong for a blue one on the same supply, even at the same target current.

The result includes the nearest E24 (5% tolerance) resistor value, rounded up rather than down or to the closest — a slightly higher resistance means slightly lower current than calculated, which is the safe direction to round in. It also recommends a power rating with headroom, since running a resistor at exactly its rated power leaves it hot and its value drifting.