LED Resistor Calculator

Find the right resistor for your LED in seconds – exact value, the next standard resistor, real current and the wattage you need.

  • Free
  • E12 values
  • LEDs in series
  • Power rating

Typical forward voltage: red 2 V, green 2–3 V, blue/white 3–3.2 V.

Standard resistor–
Exact value–
Actual current–
Power → rating–

How to calculate an LED resistor

  1. 1

    Enter the supply

    Type the supply voltage, e.g. 5 V from USB or 12 V.

  2. 2

    Enter the LED data

    Forward voltage and current come from the datasheet; 20 mA is typical for 5 mm LEDs.

  3. 3

    Use the result

    Take the standard resistor shown – it is never below the exact value, so the LED never gets too much current.

Why use this LED resistor calculator?

  • Safe standard values

    Rounds up to the next E12 resistor and shows the actual current and the wattage with 2× headroom.

  • Instant and private

    Results update as you type; nothing is sent anywhere.

  • Free, no sign-up

    No account, no ads in the way, works on phone and desktop.

  • Formulas shown

    The page explains how each result is calculated, so you can check it.

Popular uses

  • Arduino and ESP32 indicator LEDs
  • 12 V LED strips and car lighting
  • Model railways
  • Replacing bulbs with LEDs
  • Electronics kits
  • Status lights in 3D printers

Frequently asked questions

How do I calculate the resistor for an LED?

R = (supply voltage − LED forward voltage × number of LEDs) / current. For a red LED (2 V) on 5 V at 20 mA: (5 − 2) / 0.02 = 150 Ω.

What if the exact value is not a standard resistor?

Use the next higher standard value. The calculator does this for you and shows the slightly lower current that results.

Which wattage do I need?

The power is voltage drop × current. Pick a resistor rated at least twice that – usually 1/8 W or 1/4 W for single LEDs.