Question 1

How do I calculate the resistor for an LED?

Accepted Answer

Use Ohm's Law: R = (Vsupply - Vled) / Iled. Enter your supply voltage, the LED's forward voltage drop and its forward current, and the calculator returns the resistance in ohms plus the recommended wattage.

Question 2

What wattage resistor do I need?

Accepted Answer

Power dissipated in the resistor equals (Vsupply - Vled) x Iled. Always pick a resistor rated at least 2x this value for safety and longer life — the tool shows the recommended wattage automatically.

Question 3

Can I wire multiple LEDs in series?

Accepted Answer

Yes. Set the number of LEDs in series — the calculator multiplies the forward voltage by N before subtracting from the supply. The supply must be larger than N x Vled or no valid resistor exists.

Question 4

Are my calculations stored anywhere?

Accepted Answer

No. All math runs entirely in your browser using JavaScript. No inputs are sent to any server.