About the LED Resistor Calculator
What is the LED Resistor Calculator?
LEDs need a current-limiting resistor, or they draw too much current and fail. This LED resistor calculator finds the exact value, rounds up to the next standard E12 resistor and checks the resistor's power rating.
You can put several LEDs in series as long as their total forward voltage is below the supply voltage.
How to use the LED Resistor Calculator
- Supply Voltage (V) (sample: 12)
- LED Forward Voltage (V) (sample: 2)
- LED Current (mA) (sample: 20)
- LEDs in Series (sample: 3)
- Press Calculate LED Resistor (results also refresh as you type). The result card shows the Standard Resistor (E12), along with Exact resistance, Actual LED current, Power in the resistor and Resistor rating (2× margin).
Formula used
R = (V_supply − n × V_f) ÷ I_LED
- V_f = LED forward voltage: red ~2.0 V, green ~3.0 V, blue/white ~3.2 V
- Resistor power = I² × R, with a 2× safety margin
Always round up to the next standard value, which slightly reduces current.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Supply Voltage (V) | 12 |
| LED Forward Voltage (V) | 2 |
| LED Current (mA) | 20 |
| LEDs in Series | 3 |
| Result | Value |
|---|---|
| Standard Resistor (E12) | 330 Ω |
| Exact resistance | 300 Ω |
| Actual LED current | 18.18 mA |
| Power in the resistor | 109.1 mW |
| Resistor rating (2× margin) | 1/4 W |
Tips for using the LED Resistor Calculator
- Standard 5 mm LEDs are usually driven at 10–20 mA.
- Use a constant-current driver instead of resistors for high-power LEDs.