LED Array Resistor Calculator
To safely drive a series string of LEDs, you need a current-limiting resistor. The formula R = (Vs - n * Vf) / I calculates the required resistance, where Vs is the supply voltage, n is the number of LEDs in series, Vf is the forward voltage per LED (from the datasheet), and I is the desired operating current. Enter the values below to find the ideal resistor value and the minimum power rating needed. Choose the nearest standard resistor value that is equal to or greater than the calculated value.
LED series resistor formula
R = (Vs - n * Vf) / I
P = I^2 * R = (Vs - n * Vf) * I
Enter current in milliamps (mA); the calculator converts to amps internally. Example: Vs = 12 V, 3 red LEDs (Vf = 2.1 V each), I = 20 mA: R = (12 - 3*2.1) / 0.020 = (12 - 6.3) / 0.020 = 285 ohms (use 300 ohms standard).
LED circuit design tips
- Always check the LED datasheet for the maximum forward current (If_max) and do not exceed it, even briefly.
- For parallel LED strings, each string must have its own resistor to account for slight differences in forward voltage between LEDs.
- Standard resistor values follow the E12 or E24 series; always round up to the next available value to keep current at or below the target.
- Power dissipation in the resistor generates heat. For high-current applications, consider constant-current LED driver ICs instead of resistors.
Frequently asked questions
Why do LEDs need a series resistor?
LEDs have a forward voltage drop but do not inherently limit current. Without a series resistor, current rises rapidly with voltage, causing the LED to overheat and fail. The resistor limits current to the rated value (typically 10 to 30 mA for standard LEDs).
What is the LED series resistor formula?
R = (Vs - n * Vf) / I, where Vs is supply voltage, n is the number of LEDs in series, Vf is the forward voltage per LED, and I is the desired current in amperes. The numerator is the voltage remaining after the LEDs take their share.
What is a typical LED forward voltage?
Forward voltage varies by LED color: red and yellow LEDs are typically 1.8 to 2.2 V; green and blue LEDs are typically 3.0 to 3.5 V; white LEDs (which use a blue chip) are typically 3.0 to 3.6 V. Always check the datasheet for your specific LED.
What wattage resistor should I use?
Resistor power = I squared * R, or equivalently (Vs - n*Vf) * I. Use a resistor rated for at least twice this power for reliability. For example, if calculated power is 0.1 W, use a 0.25 W (1/4 W) resistor.
How many LEDs can I put in series?
The maximum number of LEDs in series is limited by the supply voltage: n_max is the largest integer where n * Vf is less than Vs, leaving enough voltage for the series resistor (at least a few tenths of a volt minimum).
Official sources
- NIST: NIST SP 811, Guide for SI Units.
- IEEE: IEEE Standards Association.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.