LED Resistor Calculator

How to use the led resistor calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

An LED is a diode, not a bulb: its current rises almost vertically once its forward voltage is reached, so connecting one directly across a supply destroys it in moments. The series resistor exists to set that current, and it is the single most common thing beginners omit.

The arithmetic is Ohm's law applied to the leftover voltage. Subtract the LED's forward voltage from the supply, then divide by the current you want: R = (Vsupply − Vf) / I.

Forward voltage varies by colour because it depends on the semiconductor. Red is typically around 2.0 V, blue and white nearer 3.2 V. That difference matters — a resistor sized for a red LED will underdrive a blue one noticeably.

The nearest standard value is given because a computed 213 Ω is not a part you can buy. Always round up rather than down: a slightly larger resistor gives slightly less brightness, while a smaller one pushes current past the rating.

Common uses

• Wiring an indicator LED to a 5 V or 12 V supply.
• Choosing a resistor for an LED on a microcontroller pin.
• Checking whether an existing resistor is safe for a brighter LED.

Frequently asked questions

What happens without a resistor?

Current is limited only by the LED's own tiny internal resistance, so it rises far past the rating and the LED fails — often within seconds.

Should I round up or down to a standard value?

Up. A larger resistor means slightly less brightness; a smaller one pushes current above the LED's rating and shortens its life.

What forward voltage should I use?

Around 2.0 V for red, 2.1 V for yellow, 2.2 V for green, and 3.2 V for blue and white. The datasheet is better than any of these if you have it.