LED resistor examples for 3.3V, 5V, 9V and 12V

Choose resistance, then check power

For one LED, R = (supply voltage − LED forward voltage) ÷ current in amperes. For several LEDs in series, subtract their combined forward voltage. Pick a standard resistor at or above the calculated value, then recalculate the current and resistor power using that resistor.

These examples use a target current of 20 mA and illustrative forward voltages of 2.0 V for red and 3.2 V for white or blue. Your LED's datasheet and desired brightness determine the actual values to use.

Supply	LED forward V	Ideal R	E12 choice	Actual current	Resistor power	Example rating
3.3 V	2.0 V	65 Ω	68 Ω	19.12 mA	0.025 W	0.25 W
5 V	2.0 V	150 Ω	150 Ω	20.00 mA	0.060 W	0.25 W
9 V	2.0 V	350 Ω	390 Ω	17.95 mA	0.126 W	0.5 W
12 V	2.0 V	500 Ω	560 Ω	17.86 mA	0.179 W	0.5 W
5 V	3.2 V	90 Ω	100 Ω	18.00 mA	0.032 W	0.25 W
12 V	3.2 V	440 Ω	470 Ω	18.72 mA	0.165 W	0.5 W

12 V worked example

A red LED with 2.0 V forward voltage leaves 10 V across the resistor. At 20 mA, the ideal resistance is 10 ÷ 0.020 = 500 Ω. Rounding up to 560 Ω gives 10 ÷ 560 = 0.01786 A, or 17.86 mA. The resistor dissipates 10² ÷ 560 = 0.179 W. A 0.5 W resistor provides more than twice that nominal dissipation.

Three LEDs in series

Three 2 V LEDs on a 12 V supply leave 6 V for the resistor. The ideal resistance at 20 mA is 300 Ω. An E12 value of 330 Ω gives about 18.18 mA and dissipates 0.109 W, so a 0.25 W resistor has more than twice the nominal power margin.

Check the real circuit

Use the highest expected supply voltage and lowest expected LED forward voltage to check maximum current and power. Resistor tolerance and hot operating conditions can require extra margin. Give each parallel LED branch its own current-limiting resistor. High-power LEDs generally need a suitable constant-current driver.

If the LED forward voltage equals or exceeds the supply, a resistor calculation cannot provide the requested current. Use a supply or driver suited to the LED string.

Choose resistance, then check power

Supply	LED forward V	Ideal R	E12 choice	Actual current	Resistor power	Example rating
3.3 V	2.0 V	65 Ω	68 Ω	19.12 mA	0.025 W	0.25 W
5 V	2.0 V	150 Ω	150 Ω	20.00 mA	0.060 W	0.25 W
9 V	2.0 V	350 Ω	390 Ω	17.95 mA	0.126 W	0.5 W
12 V	2.0 V	500 Ω	560 Ω	17.86 mA	0.179 W	0.5 W
5 V	3.2 V	90 Ω	100 Ω	18.00 mA	0.032 W	0.25 W
12 V	3.2 V	440 Ω	470 Ω	18.72 mA	0.165 W	0.5 W

12 V worked example

Check the real circuit

If the LED forward voltage equals or exceeds the supply, a resistor calculation cannot provide the requested current. Use a supply or driver suited to the LED string.