How do I calculate an LED resistor?

R = (Vsupply - Vled) / Iled. For a 5V supply with a red LED (2V forward voltage) at 20mA: R = (5 - 2) / 0.020 = 150 ohms.

What happens without a resistor?

Without a current-limiting resistor, the LED will draw too much current and burn out almost instantly. The resistor limits the current to a safe level.

LED Resistor Calculator

Find the correct resistor value to safely drive an LED. Enter supply voltage, LED forward voltage, and desired current to calculate the required resistance and power dissipation.

Inputs

Supply Voltage (V)

LED Forward Voltage (V)

Red: 1.8-2.2V, Green: 2.0-3.5V, Blue/White: 3.0-3.6V

LED Current (mA)

Standard LED: 20mA, High-power: 350mA+

LEDs in Series

Results

Resistor Needed

150 Ω

Actual Current

20.0 mA

Power (resistor)

60.0 mW

LED Resistor Results

Parameter	Value
Supply Voltage	5.0 V
Total LED Voltage Drop	2.0 V (1 × 2V)
Voltage Across Resistor	3.00 V
Desired Current	20.0 mA
Calculated Resistance	150 Ω
Nearest Standard Resistor	150 Ω
Actual Current (with standard)	20.00 mA
Power Dissipated (resistor)	60.00 mW
Power Dissipated (LED)	40.00 mW
Total Power	100.00 mW
Formula	R = (Vsupply - Vled) / Iled

Frequently Asked Questions

Related Calculators

Ohm's Law Calculator

Calculate voltage, current, or resistance using Ohm's Law (V = IR).

Voltage Divider Calculator

Calculate voltage divider output using Vout = Vin x R2/(R1+R2).

Resistor Color Code Calculator

Find resistor value from color bands (4-band and 5-band).