LED resistor calculator
Work out the current-limiting resistor for an LED from your supply voltage, the LED's forward voltage and the current you want. You get the exact value, the nearest standard (E12/E24) resistor and the wattage it needs.
Enter your values
Every LED needs a series resistor to limit its current. Enter your supply and LED, and get a standard resistor value and its wattage.
Result
R = (Vs − n × Vf) ÷ IfP = I² × RCoro's tip: Never connect an LED straight to a battery without a resistor. It can overheat in seconds. For a parallel row of LEDs, give each LED its own resistor.
LED resistor calculator: common questions
How do I calculate the resistor for an LED?
Subtract the LED forward voltage from the supply voltage and divide by the LED current: R = (Vs − Vf) ÷ If. For example (5 V − 2 V) ÷ 0.02 A = 150 Ω.
What current should a 5 mm LED run at?
Most standard 5 mm indicator LEDs are rated around 20 mA maximum; 10–20 mA is a safe range. Always check the datasheet.
Can I connect an LED without a resistor?
No. Without a resistor (or a constant-current driver) the current is not limited and the LED can overheat and fail in seconds.
For learning and low-voltage hobby circuits (12 V and below). Always check the part's datasheet. Never use these tools to plan mains (230 V) wiring.