LED Resistor Calculator

Calculate the current-limiting resistor value for an LED circuit.

Supply voltage (V)

LED forward voltage (V)

LED current (mA)

LEDs in series

Resistor value —

Nearest standard value (E24) —

Power dissipated in resistor —

Recommended resistor wattage —

R = (Vsupply − (Vled × N)) / Iled. Always use a resistor with a wattage rating at least 2× the calculated power dissipation.

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Common LED Forward Voltages

LED Color	Forward Voltage	Typical Current
Red	1.8–2.2V	20mA
Orange	2.0–2.2V	20mA
Yellow	2.0–2.2V	20mA
Green	2.0–3.0V	20mA
Blue	3.0–3.4V	20mA
White	3.0–3.4V	20mA
UV	3.1–3.5V	20mA
Infrared	1.2–1.6V	20–50mA

How to Size an LED Resistor

LEDs require a current-limiting resistor to prevent them from burning out. Without one, an LED will draw excessive current and fail almost instantly.

The formula is simple: R = (Vsupply − Vled) / Iled. For multiple LEDs in series, subtract the total LED voltage drop: R = (Vsupply − N × Vled) / Iled.

Always choose a standard resistor value equal to or greater than the calculated value. A slightly higher resistance means slightly less current and brightness, which is always safer than too little resistance.

Check the power dissipation: P = (Vsupply − Vled) × Iled. Use a resistor rated for at least twice this power to ensure long life.

The formula is R = (V_supply − V_forward) / I_forward. The forward voltage V_f is the voltage an LED "uses up" — typically 1.8–2.2V for red/yellow, 2.8–3.4V for blue/white/green. The forward current I_f is usually 20mA (0.020A) for standard 5mm LEDs, or 350mA–1A for high-power emitters.

Example: powering a red LED (V_f = 2.0V) from a 5V USB supply at 20mA: R = (5.0 − 2.0) / 0.020 = 150Ω. Choose the nearest standard resistor value (150Ω is a standard value in the E12 series).

Always round up to the next standard resistor value to stay within the LED's current rating. Running slightly below rated current extends LED life significantly — 15mA instead of 20mA is a common conservative choice.

For multiple LEDs in series, add their forward voltages: two red LEDs in series from 12V need R = (12 − 4.0) / 0.020 = 400Ω. In parallel, each LED needs its own resistor — shared resistors cause uneven brightness as LEDs age.