About

Light Emitting Diodes (LEDs) are current-driven devices. Unlike incandescent bulbs, they have very low internal resistance and will effectively short-circuit a power supply if connected directly, destroying the LED immediately. A resistor is required in series to limit the current.

This calculator determines the safe resistor value based on your power supply voltage and the specific forward voltage (Vf) of your LED. It also accounts for multiple LEDs connected in series, a common configuration in custom lighting and signage.

Formulas

For a series circuit with n LEDs:

R = V_source − (n × V_f)I_led

If the result of n × V_f is greater than V_source, the LEDs will not light up.

Reference Data

LED Color	Typical Voltage (V_f)	Current (I)
Infrared	1.2 - 1.5 V	20 mA
Red	1.8 - 2.2 V	20 mA
Orange / Yellow	2.0 - 2.2 V	20 mA
Green (Standard)	2.0 - 2.4 V	20 mA
Blue / White / UV	3.0 - 3.4 V	20 - 30 mA
High Power (1W)	3.2 - 3.6 V	350 mA

Frequently Asked Questions

It is generally not recommended. LEDs have slight manufacturing variations in Forward Voltage. If they share one resistor, the LED with the lowest voltage drop will "hog" the current, potentially burning out, causing a cascade failure of the others. Always use one resistor per LED (or per series string).

Standard resistors are 1/4 Watt (0.25W). This tool calculates the actual power. If your power is above 0.25W, you must use a larger resistor (e.g., 0.5W or 1W) to prevent overheating.