What is the Vf of an LED?

Vf is the forward voltage: the voltage drop across the LED when it conducts. It depends on the type: red around 2V, blue and white around 3.2-3.4V.

Why does an LED need a series resistor?

An LED is a constant-voltage device: with no current limiting it draws effectively unlimited current and burns out. The resistor sets the current to the value you want.

LED resistor calculator

Formulas, limitations and content checked according to our Editorial method.

Calculate the series resistor for driving an LED from supply voltage, forward voltage (Vf) and target current. Shows the ideal value and the nearest E12 value.

Single LED or series LEDs Standard E12 value Dissipated power

Parameters

Supply voltage Vcc (V)

LED forward voltage Vf (V)

LED current If (mA)

LED in series (N)

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FAQ

What happens without resistor? The LED draws current limited only by its internal resistance, which is very low. In a few milliseconds it shorts out and burns out.

Can I use the lower or higher E12 value? Lower → more current → more brightness, but pay attention to the power and life of the LED. Higher → less current → less brightness.

Internal links

Color coded resistor To find the value of the resistor you have in your hand.

S/P resistors Combine multiple resistors to get the desired value.

Practical LED examples

Arduino / 5V

Red LED 2 V at 10 mA: ideal value approximately 300 Ω. In E12 330 Ω is better, less current and less crying.

Why choose above

The lower E12 value increases the current. For prototypes and indicator LEDs, the higher value is almost always the most prudent choice.

Power

Always check the power dissipated. If the calculator reports more than 0.25 W, avoid the classic tiny resistor thrown out there on faith.

Typical Vfs to use as a starting point

LEDs	Vf indicative	Notes
Red	1.8-2.2V	Good for low current indicators.
Green	2.0-3.2V	It depends a lot on technology.
Blue	3.0-3.4V	With 5 V there is little margin left on the resistance.
White	3.0-3.4V	Similar to blue, often with phosphors.

Frequently asked questions

What resistor for a 5V LED?

With Vf=2V, If=20mA: R = (5−2)/0.02 = 150Ω. Nearest E12 value: 150Ω.

What resistor for a 12V LED?

With Vf=2V, If=20mA: R = (12−2)/0.02 = 500Ω. Nearest E12: 470Ω or 560Ω.

How do I calculate resistor power for an LED?

P = (Vcc − Vf) × If. For 5V, Vf=2V, 20mA: P = 60mW. Use a 0.25W resistor.

Can I put LEDs in series?

Yes: R = (Vcc − N×Vf) / If. Three red LEDs (2V each) at 12V: R = (12−6)/0.02 = 300Ω.