How to use the LED resistor calculator
- Choose your LED colour — this fills in a typical forward voltage and current. Pick Custom to type values from the datasheet.
- Enter the supply voltage — 5 V for an Arduino Uno, 3.3 V for an ESP32, 9 V for a battery, 12 V for an adapter.
- Set how many LEDs are in series (connected end to end).
- Read the standard resistor value, its colour bands, the real LED current and how much power the resistor must handle.
The formula
The resistor takes up whatever voltage the LEDs do not use, and sets the current:
- V_supply – supply voltage
- n – number of LEDs in series
- V_f – forward voltage of one LED (from the datasheet)
- I_LED – the current you want, in amps (15 mA = 0.015 A)
The resistor's power is P = (V_supply − n × V_f) × I_LED.
Typical LED forward voltages
| LED colour | Typical V_f | Typical current |
|---|---|---|
| Red | 1.8 – 2.2 V | 10 – 20 mA |
| Yellow / Orange | 2.0 – 2.2 V | 10 – 20 mA |
| Standard green | 2.0 – 2.4 V | 10 – 20 mA |
| Bright / true green | 2.8 – 3.4 V | 10 – 20 mA |
| Blue / White | 2.9 – 3.4 V | 10 – 20 mA |
| Infrared | 1.2 – 1.5 V | 20 – 50 mA |
These are typical values for small 3 mm and 5 mm LEDs. Always check the datasheet for your exact part, and never use power LEDs (1 W and above) with only a resistor — they need a constant-current driver.
Why round up to a standard value?
Resistors are sold in standard series. E12 has 12 values per decade (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82) and E24 has 24. Rounding up always gives slightly less current, which keeps the LED within its rating.
Series or parallel?
Several LEDs in series can share one resistor, and waste less power. LEDs in parallel should each get their own resistor, because no two LEDs are identical and one would take most of the current.
Want the theory behind this? Read Ohm's law explained with examples, or calculate other values with the Ohm's law calculator.
