LED Resistor Calculator

Find the correct resistor value and wattage to safely drive an LED from any supply voltage.

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1. Select the LED color from the presets (red, green, blue, white, etc.) or enter a custom forward voltage. 2. Enter your power supply voltage in the Vsupply field. 3. Set the desired LED current (typically 20 mA for standard LEDs). 4. Choose single LED, series, or parallel configuration and set the number of LEDs if using multiple. 5. View the calculated resistor value, nearest standard resistor, and required wattage rating. 6. Verify that your supply voltage is sufficient for the total forward voltage drop in series configurations.

About This Tool

Driving an LED requires a current-limiting resistor to prevent the LED from drawing too much current and burning out. This calculator determines the correct resistor value based on your supply voltage, LED forward voltage, and desired LED current. It also recommends the nearest standard resistor value and calculates the power the resistor will dissipate so you can choose an appropriately rated component.

The tool supports single LED setups as well as series and parallel configurations of multiple LEDs. For series chains, it sums the forward voltage drops and checks whether the supply voltage is sufficient. For parallel arrays, it calculates the total current draw and individual resistor values.

Common LED forward voltages are pre-loaded for red, orange, yellow, green, blue, white, and UV LEDs, but you can enter any custom value. Typical forward currents of 20 mA for standard LEDs and 350 mA or more for high-power LEDs are supported, making this tool useful for everything from indicator lights to LED lighting projects.

Formula / How It Works

R = (Vsupply - Vforward) / I | Power = (Vsupply - Vforward) x I | For series LEDs: R = (Vsupply - n x Vforward) / I

Frequently Asked Questions

LEDs are current-driven devices with very low internal resistance. Without a resistor, an LED connected directly to a voltage source will draw excessive current, overheat, and burn out almost instantly. The resistor limits the current to a safe level specified in the LED datasheet, typically 20 mA for standard LEDs.
Use the formula R = (Vsupply - Vforward) / I, where Vsupply is your power source voltage, Vforward is the LED forward voltage drop (varies by color), and I is the desired current (typically 0.020 A for standard LEDs). Then choose the nearest higher standard resistor value.
Typical forward voltages vary by LED color: red 1.8-2.2V, orange 2.0-2.2V, yellow 2.0-2.2V, green 2.0-3.5V (depending on type), blue 3.0-3.5V, white 3.0-3.5V, and UV 3.3-3.8V. Always check the datasheet for your specific LED as values can vary by manufacturer.
You can use one resistor for LEDs in series because they share the same current. Add up all the forward voltages and subtract from the supply voltage to calculate the resistor. For LEDs in parallel, each branch should have its own resistor because LEDs have slightly different forward voltages and will not share current equally.
Calculate the power dissipated by the resistor using P = I^2 x R or P = (Vsupply - Vforward) x I. Then choose a resistor rated for at least twice the calculated power to provide a safety margin. For most standard LED circuits (20 mA), a 1/4 watt resistor is sufficient.

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