Electronics
LED resistor calculator
Voltage, Vf and current → series resistor and power for your LED.
- Instant
- Free
- Private (processed locally)
- No sign-up
Size an LED resistor
Enter the supply voltage, the LED forward voltage Vf and the desired current. The tool computes the series resistor and the power it dissipates.
The formula
- R = (Vsupply − Vf) / I
- I in amperes (20 mA = 0.02 A)
- P = (Vsupply − Vf) × I
- Pick the next higher standard value
Example
| Input | Value |
|---|---|
| Supply / Vf / I | 5 V / 2 V / 20 mA |
| Resistor | 150 Ω |
| Power | 0.06 W |
100% local calculation, no data sent.
Frequently asked questions
What is the formula?
R = (Vsupply − Vf) / I, with I in amperes. The power in the resistor is P = (Vsupply − Vf) × I.
Which current should I pick?
Many standard LEDs run around 20 mA, but check the datasheet: some high-brightness LEDs differ.
What is Vf?
The forward voltage depends on color: ~1.8–2.2 V (red), ~3–3.4 V (blue/white). See the datasheet.
Which resistor value should I buy?
Take the next higher standard value (E12/E24) and make sure its power rating exceeds the dissipated power.