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LED Resistor Calculator: Choose the Right Resistor

This free LED resistor calculator finds the resistor you need to run an LED safely from a given supply. Enter the voltage, the LED’s forward voltage and the current, and read the value, the power and the nearest standard resistor.

By Debmedia Technologies LLP · Kolkata, India · Updated

What Is It and When Do You Need It?

An LED needs a resistor to limit the current. The right value keeps the LED bright and long-lived. The calculation is short, but the power rating and the standard values are easy to forget.

Enter the supply, the forward voltage and the current.

How to Use This Tool

  1. Enter the supply voltage.
  2. Enter the LED forward voltage and the current in mA.
  3. Optionally enter the number of LEDs in series.
  4. Press Calculate.

How the Calculation Works

The voltage across the resistor is the supply minus the total forward voltage of the LEDs in series. R = that voltage ÷ current. Power = voltage across the resistor × current. The tool picks the next higher value from the E12 series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their tens multiples) and shows the real current with it.

Worked Example

For a 9 V supply, a 2 V LED and 20 mA, R = 350 Ω. The next E12 value is 390 Ω, giving 17.9 mA, and the resistor dissipates 0.14 W.

Common Uses

  • Hobby electronics.
  • Arduino and Raspberry Pi projects.
  • Indicator lights.
  • Learning basic circuits.

Tips and Common Mistakes

  • Use a ¼ W resistor for most small LEDs.
  • Lower current is safer and still bright.
  • Check the LED’s maximum current.

Next, you may need the Ohm’s law calculator, resistor color code calculator, AWG wire gauge calculator, electricity cost calculator.

Everything runs in your browser. Nothing you type is uploaded, stored or sent to any outside service.

Frequently Asked Questions

An LED does not limit its own current. Without a resistor it would draw too much current and burn out.
R = (supply voltage − LED forward voltage) ÷ current. For 9 V, a 2 V LED and 20 mA, R = 7 ÷ 0.02 = 350 Ω.
About 1.8 to 2.2 V for red, yellow and green, and 3.0 to 3.4 V for blue and white. Check the data sheet.
Choose the next higher standard value, so the current is slightly lower than the target. The tool shows it.

Disclaimer: results are calculated in your browser from the numbers you enter. Check important results before you rely on them for money, health, safety or legal decisions.