LED Resistor Calculator
Get the current-limiting resistor and the power rating it needs from your supply voltage and LED specs.
Formula
R = (Vs − n·Vf) / I. An exact match is rare, so pick the next standard value up. The current drops a little, which is safer for the LED. Check the LED datasheet for its actual forward voltage.
Worked example: a red LED on 5 V at 20 mA
- Voltage across the resistor: 5 V − 2.0 V = 3 V
- Resistance: 3 V ÷ 0.02 A = 150 Ω (a standard E24 value, 150 Ω)
- Power: 3 V × 20 mA = 60 mW → with 2× margin a 1/4 W resistor is plenty
LED resistor chart by supply voltage (20 mA)
| Supply | Red (2 V) | Green / blue (3 V) | White (3.2 V) |
|---|---|---|---|
| 3.3 V | 68 Ω | Too little headroom | Too little headroom |
| 5 V | 150 Ω | 100 Ω | 91 Ω |
| 9 V | 360 Ω | 300 Ω | 300 Ω |
| 12 V | 510 Ω | 470 Ω | 470 Ω |
For 20 mA, rounded up to the next E24 value. With less than 0.8 V across the resistor, LED-to-LED forward voltage spread makes brightness unpredictable, so it is not recommended.
Frequently asked questions
What happens if I connect an LED without a resistor?
Above its forward voltage, an LED's current rises very steeply. Without something limiting the current, it overheats and fails quickly. Always use a resistor or a constant-current driver.
Can several LEDs in parallel share one resistor?
Forward voltages vary slightly from LED to LED, so current crowds into one of them. Give each parallel LED its own resistor.