An LED resistor calculator determines the current-limiting resistor needed to safely power an LED from a given voltage source. Without the correct resistor, LEDs draw excessive current and burn out quickly. Enter your supply voltage, LED forward voltage, and desired current to get the exact resistor value, the nearest standard E24 value, and the recommended power rating.
Circuit Parameters
Calculation Details
Circuit Diagram
LED Forward Voltage Reference
| LED Color | Typical Vf | Typical If | Wavelength |
|---|---|---|---|
| Red | 1.8V | 20mA | 620-750nm |
| Orange | 2.0V | 20mA | 590-620nm |
| Yellow | 2.1V | 20mA | 570-590nm |
| Green | 2.2V | 20mA | 495-570nm |
| Blue | 3.2V | 20mA | 450-495nm |
| White | 3.3V | 20mA | Broad spectrum |
| UV | 3.4V | 20mA | 10-400nm |
| IR | 1.2V | 20mA | 750nm-1mm |
How to Use the LED Resistor Calculator
Every LED circuit needs a current-limiting resistor to prevent the LED from drawing too much current and burning out. This LED resistor calculator takes the guesswork out of component selection by computing the exact resistance, the nearest standard value, and the minimum power rating your resistor needs.
Step 1: Set Your Supply Voltage
Enter the voltage of your power source. Use the quick preset buttons for common voltages like 3.3V (microcontrollers), 5V (USB/Arduino), 9V (battery), 12V (automotive/LED strips), or 24V (industrial). You can also type any custom voltage.
Step 2: Choose Your LED Type
Click an LED color preset to auto-fill the forward voltage and current, or enter custom values. Standard LEDs typically operate at 20mA, while high-power LEDs may need 350mA or more. Check your LED's datasheet for exact values. The forward voltage varies by color: red LEDs are around 1.8V, while blue and white LEDs are around 3.2-3.3V.
Step 3: Set the Number of LEDs
If you are wiring multiple LEDs in series (positive to negative in a chain), increase the LED count. The calculator adjusts the formula to account for the total voltage drop across all LEDs. Make sure your supply voltage exceeds the sum of all forward voltages, or the circuit will not work.
Step 4: Read the Results
The calculator shows four key values: the calculated resistance using R = (Vsupply - N x Vf) / If, the nearest standard E24 resistor value, the power dissipated by the resistor, and the minimum recommended wattage rating. A simple circuit diagram illustrates the component arrangement.
Understanding the E24 Standard Series
Resistors are manufactured in standard value series. The E24 series provides 24 values per decade (e.g., 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30...) with 5% tolerance. This LED resistor calculator always rounds up to the next E24 value, which slightly reduces current and provides a safety margin. For most LED circuits, a standard 1/4W carbon film resistor from the E24 series is ideal.
Tips for Reliable LED Circuits
Always choose a resistor wattage rating at least double the calculated power dissipation for long-term reliability. When running LEDs from batteries, account for voltage sag as the battery discharges. For multiple LEDs, series wiring shares one resistor but requires higher voltage, while parallel wiring needs one resistor per LED. All calculations happen in your browser and your data stays completely private.
Frequently Asked Questions
Is this LED resistor calculator free?
Yes, this LED resistor calculator is 100% free with no usage limits. Calculate as many resistor values as you need without creating an account or paying anything.
Is my data safe when using this tool?
Absolutely. All calculations run entirely in your browser using JavaScript. No data is sent to any server or stored anywhere. Your circuit values never leave your device.
Why do I need a resistor for an LED?
LEDs have a very low internal resistance and will draw excessive current without a limiting resistor, which can burn out the LED or damage your circuit. The resistor limits the current to the LED's safe operating value, typically 20mA for standard LEDs.
What is the formula for calculating an LED resistor?
The formula is R = (Vsupply - Vforward) / Iforward. Vsupply is your power source voltage, Vforward is the LED's forward voltage drop, and Iforward is the desired operating current. The result is the minimum resistance needed in ohms.
What is the nearest standard resistor value?
Standard resistors come in specific values defined by the E24 series (24 values per decade with 5% tolerance). The calculator finds the nearest E24 value that is equal to or higher than the calculated resistance, ensuring your LED gets the correct or slightly less current.
Can I use this calculator for LEDs in series?
Yes. Set the number of LEDs in series and the calculator adjusts the formula to R = (Vsupply - N x Vforward) / Iforward, where N is the number of LEDs. Make sure your supply voltage is higher than the total forward voltage of all LEDs combined.
How do I choose the right wattage rating for my resistor?
The calculator computes power dissipation using P = (Vsupply - Vforward) x Iforward and recommends the next standard wattage rating (1/8W, 1/4W, 1/2W, or 1W). Always use a resistor rated at least double the calculated dissipation for reliable operation.
What is a typical LED forward voltage?
Forward voltage depends on the LED color. Red LEDs typically have 1.8V, yellow 2.1V, green 2.2V, blue 3.2V, and white 3.3V. UV LEDs run around 3.4V and infrared LEDs around 1.2V. These values can vary by manufacturer, so check your LED's datasheet.