What LED forward voltage should I enter?

Use the value from the LED datasheet. Typical drops are about 1.8-2.2 V for red, yellow, and green LEDs, and 3.0-3.4 V for blue, white, and many high-brightness types.

Why does the supply voltage have to exceed the LED voltage?

The resistor can only drop the voltage left over after the LED. If the supply equals or is below the forward voltage there is nothing for the resistor to limit, so the calculator requires the supply to be higher.

Why pick the nearest E12 value instead of the exact ohms?

Resistors are sold in standard decade series such as E12. Choosing a slightly higher standard value lowers current a touch and keeps the LED safely below its maximum rating while letting you buy an off-the-shelf part.

What resistor power rating do I need?

Pick a rating comfortably above the dissipated power the calculator reports. Small indicator LEDs at 10-20 mA usually dissipate well under a quarter watt, so a common 0.25 W resistor is adequate.

LED Resistor Calculator

Supply Voltage

LED Forward Voltage

LED Current

Series Resistor350.00 Ω

Nearest E12 Value330 Ω

Resistor Power0.1400 W

The LED Resistor Calculator sizes the series resistor that protects a light-emitting diode from excess current. Because an LED behaves like a fixed voltage drop rather than a resistance, it needs a resistor to soak up the difference between your supply rail and the LED forward voltage. Enter the supply voltage, the LED forward voltage, and the current you want, and the tool returns the exact resistance, the power the resistor must dissipate, and the closest standard E12 value to buy.

Formula

R = (Vsupply − Vled) / I; Presistor = (Vsupply − Vled) × I

Vsupply: Supply voltage feeding the LED branch (V)
Vled: LED forward voltage drop (V)
I: Desired LED forward current (A)
R: Required series resistor (ohms)

How it works

Enter the supply voltage feeding the circuit, the LED forward voltage drop from its datasheet (around 1.8 V for red, 3.0-3.4 V for blue and white), and the target forward current in milliamps.
The calculator subtracts the LED forward voltage from the supply voltage to find the voltage that must drop across the resistor, then divides by the current in amps to get the resistance in ohms.
It also reports the power the resistor dissipates and rounds the ideal resistance to the nearest E12 series value so you can pick a real part.

Worked example

Driving a standard LED with a 2 V forward drop at 20 mA from a 9 V supply.

Voltage across the resistor = 9 − 2 = 7 V.
Resistance = 7 ÷ 0.020 A = 350 Ω.
Power = 7 × 0.020 = 0.14 W, so a quarter-watt resistor is fine.
Nearest E12 standard value to 350 Ω is 330 Ω.

Use a 330 Ω resistor (350 Ω ideal) rated at least 0.25 W.

Frequently asked questions

What LED forward voltage should I enter?: Use the value from the LED datasheet. Typical drops are about 1.8-2.2 V for red, yellow, and green LEDs, and 3.0-3.4 V for blue, white, and many high-brightness types.
Why does the supply voltage have to exceed the LED voltage?: The resistor can only drop the voltage left over after the LED. If the supply equals or is below the forward voltage there is nothing for the resistor to limit, so the calculator requires the supply to be higher.
Why pick the nearest E12 value instead of the exact ohms?: Resistors are sold in standard decade series such as E12. Choosing a slightly higher standard value lowers current a touch and keeps the LED safely below its maximum rating while letting you buy an off-the-shelf part.
What resistor power rating do I need?: Pick a rating comfortably above the dissipated power the calculator reports. Small indicator LEDs at 10-20 mA usually dissipate well under a quarter watt, so a common 0.25 W resistor is adequate.

Related calculators

Ohm's Law Calculator Voltage Divider Calculator Resistor Calculator Voltage Drop Calculator