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LED Resistor Calculator
Calculate the series resistor for an LED from the supply voltage, LED forward voltage and current, with the next standard E24 value and the power rating needed.
How to use it
- Enter your supply voltage (battery, USB 5 V, 12 V car or other).
- Enter the LED forward voltage from its datasheet (red about 2.0 V, yellow 2.1 V, green 2.2-3.0 V, blue and white 3.0-3.4 V).
- Enter the LED current - 20 mA is a common maximum; 10-15 mA is brighter than it looks and extends life.
- Use the next standard resistor value at or above the result.
How it works
R = (V supply − n × V forward) ÷ I. The resistor dissipates P = I² × R watts; pick a resistor rated at least double that. Using the next higher standard value (E24 series) keeps the LED at or below its target current.
The supply must be higher than the sum of the LED forward voltages. LEDs in parallel should each have their own resistor, because small differences in forward voltage make one LED hog the current.
FAQ
What resistor do I need for an LED on 5V?
For a typical red LED (2.0 V) at 20 mA: (5 − 2.0) ÷ 0.020 = 150 Ω. A 330 Ω resistor gives about 9 mA and is the common safe choice for Arduino and breadboard projects.
What resistor for an LED on 12V?
A red LED at 20 mA on 12 V needs (12 − 2.0) ÷ 0.02 = 500 Ω; the next standard value is 510 Ω, rated at least 0.5 W.
Can I connect an LED without a resistor?
No. An LED has almost no resistance once it conducts, so without a series resistor or constant-current driver it will draw excess current and burn out.
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