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LED Resistor Calculator

Quickly calculate the required series resistor for your LED. Find the nearest standard resistor value and power dissipation to safely light up your project.

⚙️ Parameters

This is the voltage drop across the LED.

Typically 15-20mA for standard 5mm LEDs.

đź“Š Results

Required Resistor:

Nearest Standard Resistor (E24):

Power Dissipation:

⚡ Circuit Diagram

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Theory & Formulas

To prevent an LED from burning out, a series resistor is needed to limit the current flowing through it. The calculation is a direct application of Ohm's Law.

Ohm's Law for LEDs

The resistor's value is determined by the source voltage (VS), the LED's forward voltage (VF), and the desired forward current (IF).

R = (VS - VF) / IF

Where IF is in Amperes (so we divide the milliamp value by 1000).

Power Dissipation

It's also crucial to use a resistor with an adequate power rating. The power dissipated by the resistor is calculated using the formula:

P = IF² * R

If the power is over 0.25W, you should consider a resistor with a higher power rating (e.g., 1/2W or 1W).

Frequently Asked Questions

Why do I need a resistor for an LED?

LEDs are diodes that have a very low internal resistance. Without a resistor to limit the current, they would draw an excessive amount from the power source, causing them to burn out almost instantly. The resistor ensures the current stays at a safe level.

How do I choose the resistor's power rating (wattage)?

The power rating depends on how much energy the resistor needs to dissipate as heat. Our calculator shows this as "Power Dissipation". Always choose a resistor with a power rating higher than the calculated value. For most small LEDs, a standard 1/4W resistor is sufficient, but it's always critical to check.

What happens if my source voltage is lower than the LED's forward voltage?

If the source voltage (VS) is less than the LED's forward voltage (VF), the LED will not light up. There isn't enough electrical pressure to push the current across the diode's junction.