LED Series Resistor Calculator
An LED needs a series resistor to limit its current, since an LED on its own draws whatever current the supply allows and burns out. Enter the supply voltage, the LED forward voltage, and the current you want through the LED. The calculator finds the required resistor value and the power it must dissipate. The resistor drops the difference between the supply and the LED forward voltage at the chosen current.
LED resistor formula
R = (Vsupply - Vled) / I
Resistor power P = (Vsupply - Vled) * I = I2 * R
where I = LED current (A)
The resistor drops the surplus voltage at the LED current. Always choose a resistor rated above the calculated power, commonly at least double, for headroom.
Worked example
A red LED (forward voltage 2 V) runs from a 5 V supply at 20 mA (0.02 A). R = (5 - 2) / 0.02 = 3 / 0.02 = 150.00 ohms. Power = (5 - 2) * 0.02 = 0.06 W, so a standard quarter-watt resistor is fine.
LED series resistor: frequently asked questions
How do I size an LED series resistor?
Subtract the LED forward voltage from the supply voltage, then divide by the desired LED current: R = (Vsupply - Vled) / I. This resistor drops the leftover voltage so the LED current stays at the chosen value.
What forward voltage should I use?
Forward voltage depends on the LED colour and chemistry: roughly 1.8 to 2.2 V for red, around 2.0 to 2.2 V for yellow, and 3.0 to 3.4 V for blue, green, and white LEDs. Use the figure from your LED's datasheet; the field is editable.
How much power does the resistor dissipate?
Power = (Vsupply - Vled) times I, the voltage dropped across the resistor multiplied by the current. Choose a resistor rated comfortably above this, commonly at least double, to avoid overheating.
What if the supply voltage is below the LED forward voltage?
If the supply is less than the LED forward voltage the LED cannot light at the chosen current and no positive resistor value solves the circuit. The calculator flags this case.
Sources
- NIST: SI units (volt, ampere, ohm, watt).
- The series resistor value follows directly from Ohm's law. LED forward voltages are nominal, editable to match your datasheet.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.