LED Resistor Calculator
Fill in the values and the answer updates as you type.
How the LED Resistor Calculator works
R = (supply voltage − total LED forward voltage) ÷ LED current. Power in the resistor = I² × R. Pick the next higher standard resistor value and a wattage rating at least twice the calculated power. A dash means the supply is too low for that many LEDs. Example: with the default values, resistor value comes to 500 Ω.
Inputs and results
The LED Resistor Calculator takes 4 inputs and gives 3 results.
What you enter
- Supply voltage (V) — starts at 12.
- LED forward voltage (V) — starts at 2.
- LED current (mA) — starts at 20.
- LEDs in series — starts at 1.
What you get
- Resistor value (Ω)
- Resistor power (W)
- Recommended rating, 2× margin (W)
Example: LED Resistor Calculator with the starting values
With supply voltage 12 V, led forward voltage 2 V, led current 20 mA and leds in series 1, the calculator returns:
| Resistor value (Ω) | 500 |
|---|---|
| Resistor power (W) | 0.2 |
| Recommended rating, 2× margin (W) | 0.4 |
Change any value above and the results update straight away.
LED Resistor Calculator FAQ
How do I use the LED Resistor Calculator?
Enter supply voltage, led forward voltage, led current and leds in series in the fields, then read resistor value (ω), resistor power (w) and recommended rating, 2× margin (w) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the LED Resistor Calculator calculate?
Series resistor value for an LED. It returns resistor value (ω), resistor power (w) and recommended rating, 2× margin (w) from the 4 values you provide.
What values does the LED Resistor Calculator start with?
It opens with supply voltage 12 V, led forward voltage 2 V, led current 20 mA and leds in series 1 so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the LED Resistor Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the LED Resistor Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.