LED Resistor Calculator

An LED resistor calculator works out the series (current-limiting) resistor an LED needs — its resistance and the wattage to handle — from your supply voltage, the LED's forward voltage and the current you want through it. Without that resistor, an LED on too high a voltage burns out fast.

Your measurements

Result

It also picks the nearest standard (E12) resistor value at or above the exact figure, and the power rating to use, so you can grab the right part from a kit.

How it is calculated

Resistor R = (supply voltage − forward voltage × number of LEDs) ÷ current (in amps). Power dissipated = voltage across the resistor × current; pick a resistor rated at least twice that. The nearest standard value is taken from the E12 series at or above R. LEDs in series share one resistor and add their forward voltages.

Steps

Enter the supply voltage powering the LED.
Enter the LED's forward voltage (≈2 V red, ~3 V blue/white — check the datasheet).
Enter the target current (20 mA is typical).
If wiring LEDs in series, enter how many.
Read the resistor value, the nearest standard size and the wattage.

Worked example

A 12 V supply driving one 2 V LED at 20 mA needs a 500 Ω resistor (use 560 Ω, 1/4 W).

Common questions

What resistor do I need for an LED?

R = (supply − LED forward voltage) ÷ current in amps. For 12 V, a 2 V LED at 20 mA, that's 500 Ω — round up to 560 Ω.

What wattage resistor?

Calculate power = volts across the resistor × current, then use at least double. Small indicator LEDs are usually fine on a 1/4 W resistor.

Can I run LEDs without a resistor?

Not safely on a fixed voltage — the LED draws too much current and fails. Use a resistor, or a constant-current driver for power LEDs.

How do I wire several LEDs?

In series they share one resistor and their forward voltages add (so the supply must exceed the total). In parallel, give each LED its own resistor.