LED Resistor Calculator
Find the current-limiting resistor for one or more series LEDs from supply voltage, forward voltage and current
LED circuit
Forward voltage varies with current and part; use your datasheet value for accuracy.
Number of LEDs wired in series on one resistor (each adds its forward voltage).
Forward-voltage presets are representative typical values; real LEDs vary with current, binning and manufacturer. The wattage rating includes a 2× safety margin. Always confirm with the LED datasheet.
About this tool
This LED resistor calculator sizes the series current-limiting resistor every LED needs. From the supply voltage, the LED forward voltage and the desired forward current it applies Ohm's law to find the resistor value, then gives you the nearest standard E12 and E24 parts, the resistor color bands, the recommended power rating and the actual current you would get with each standard value. You can pick the forward voltage from a color preset (red, green, blue, white and more) or type a custom value straight from your datasheet, and you can wire several LEDs in series on a single resistor. It is built for makers, hobbyists, electronics students and anyone wiring up LEDs on a breadboard, Arduino or microcontroller project.
How to use
- 1 Enter the supply voltage that will drive the LED circuit (for example 5 V from USB or 3.3 V from a microcontroller).
- 2 Set the LED forward voltage: choose a color preset or switch to custom and type the value from the datasheet.
- 3 Choose the forward current in milliamps (20 mA is typical for standard LEDs) and the number of LEDs wired in series.
- 4 Read the required resistance, the nearest E12 and E24 standard values with their color bands, the recommended wattage and the LED and resistor power.
How it works
The resistor must drop the voltage left over after the LEDs, at the design current. With n LEDs in series the resistor sees Vs − n·Vf, so by Ohm's law the value is R = (Vs − n·Vf) / If, where If is converted from milliamps to amps. If n·Vf is greater than or equal to the supply voltage there is no headroom and the LEDs cannot light, so the calculator flags the supply as too low. The resistor dissipates P_R = (Vs − n·Vf)·If = If²·R, and the LEDs together dissipate n·Vf·If. The recommended power rating is the next standard size (1/8, 1/4, 1/2, 1, 2 W) above twice the dissipation, giving a comfortable 2× margin. The nearest E12 (±10%) and E24 (±5%) standard values are picked as the closest catalog parts, and the resistor color bands are generated for the E24 value using the standard IEC 60062 color code.
Frequently asked questions
What resistor do I need for an LED?
Use R = (Vs − Vf) / If. For a single LED on 5 V with a red LED (Vf ≈ 1.8 V) at 20 mA, R = (5 − 1.8) / 0.02 = 160 Ω, so the nearest standard part is 150 Ω or 180 Ω. This calculator works it out for you including multiple series LEDs and shows the closest E12 and E24 values.
What forward voltage should I use for each color?
Typical values are red 1.8 V, orange 2.0 V, yellow 2.1 V, green 2.2 V, blue and white 3.2 V, violet/UV 3.4 V and infrared 1.2 V. These are representative; real parts vary, so for best accuracy enter the exact forward voltage from your LED's datasheet using the custom option.
What current should I run an LED at?
Standard 3 mm and 5 mm LEDs are usually rated around 20 mA, and 10–20 mA already gives plenty of brightness. Lower current (e.g. 5–10 mA) saves power and extends life; never exceed the maximum forward current in the datasheet. The presets offer 10, 20 and 30 mA, or type any value.
Can I put several LEDs on one resistor?
Yes, if they are wired in series: their forward voltages add up, so set the LED count and the calculator uses Vs − n·Vf. The supply must exceed the total forward voltage. LEDs wired in parallel should each have their own resistor instead, because small differences make them share current unevenly.
What power rating resistor should I choose?
Pick a rating above the dissipated power with margin. For example a resistor dropping 3.2 V at 20 mA dissipates 64 mW, so a 1/8 W (125 mW) part is fine, though a 1/4 W is a safe common choice. This tool already applies a 2× safety margin and recommends the next standard size.
Related tools and uses
Pair this with the Ohm's law calculator for V, I, R and power, the resistor color code calculator to read or build the bands, and the series and parallel resistance calculator for larger LED arrays.