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
| Field | What to enter |
|---|---|
| Supply voltage | Units: V, mV, kV |
| LED forward voltage | Choose one: Red (≈2.0 V), Yellow (≈2.1 V), Green (≈2.2 V), Blue / white (≈3.2 V) |
| LED current | Units: 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) |
|---|---|
| 2.5 | 25 |
| 3.75 | 87.5 |
| 5 | 150 |
| 6.25 | 212.5 |
| 7.5 | 275 |
| 10 | 400 |
Formula
R = (Vsupply − Vforward) ÷ IWorked 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.
Frequently asked questions
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.