# LED Resistor Calculator

URL: https://speedycalculator.com/calculators/physics/led-resistor/
Last updated: 2026-10-25

> The LED resistor calculator finds the series resistor needed to run an LED at a chosen current from a supply voltage, and the power the resistor dissipates.

## Inputs

- Supply voltage (V, mV, kV)
- LED forward voltage
- LED current (mA, A)

## Results

- Resistor value
- Resistor power

## Formula

```
R = (Vsupply − Vforward) ÷ I
```

## Explanation

LEDs need current limiting. Without a resistor, current rises until the LED fails.

### How to use the LED Resistor Calculator

- **Supply voltage**
- **LED forward voltage**
- **LED current**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Subtract the LED’s forward voltage from the supply and divide by the desired current.

### Inputs explained

FieldWhat to enterSupply voltageUnits: V, mV, kVLED forward voltageChoose one: Red (≈2.0 V), Yellow (≈2.1 V), Green (≈2.2 V), Blue / white (≈3.2 V)LED currentUnits: mA, A

### What each result means

- **Resistor value** = ([Supply voltage] - [LED forward voltage]) ÷ [LED current]
- **Resistor power** = ([Supply voltage] - [LED forward voltage]) × [LED current]

### Example values

How the main result changes when **Supply voltage** changes and everything else stays at the example values above:

Supply voltage (V)Resistor value (Ω)2.5253.7587.551506.25212.57.527510400

## Worked example

Inputs — Supply voltage: 5 V; LED forward voltage: Red (≈2.0 V); LED current: 20 mA.

Results — **Resistor value = 150 Ω**; Resistor power = 0.06 W.

## FAQ

**Should I round up or down?**
Round up to the next standard value to keep current at or below the target.

**What wattage resistor do I need?**
Choose at least twice the calculated power.

**What formula does the LED Resistor Calculator use?**
R = (Vsupply − Vforward) ÷ I.