# LED Series Resistor Calculator

Compute the current-limiting resistor for LEDs in series with power, nearest standard value and wattage rating

> Canonical page: https://elysiatools.com/en/tools/led-resistor-calculator

- **Category:** Math & Numbers

- **Keywords:** led resistor, led current limiting, series resistor, led, forward voltage, forward current, electronics, circuit, e12, e24

## Overview

An LED needs a series resistor to limit forward current.

Equations:
- Resistance: R = (Vs − n·Vf) / If
- Resistor power: P_R = If² · R  (pick a wattage rating ≥ 2×P_R)
- LED power: P_LED = n · Vf · If

The tool snaps the ideal resistor to the nearest E12/E24 standard value and recomputes the actual current.

## Inputs

- **Supply Voltage (Vs)** (number): e.g. 5
- **LED Forward Voltage (Vf)** (number): e.g. 2.1 (red ~1.8, green ~2.1, blue/white ~3.2)
- **LED Forward Current (If)** (number): e.g. 0.02 (20 mA)
- **Number of LEDs in Series** (number): e.g. 1
- **Standard Resistor Series** (select)

## When to use

- Designing custom LED indicator circuits on breadboards or printed circuit boards.
- Selecting the correct standard resistor value from E12 or E24 series kits for prototyping.
- Determining the minimum power rating (wattage) for a resistor to avoid thermal failure.

## How it works

- Enter the supply voltage, the forward voltage of your specific LED, and the target forward current.
- Specify the number of LEDs connected in series and choose either the E12 or E24 standard resistor series.
- The calculator computes the ideal resistance using Ohm's law, snaps it to the nearest standard resistor value, and calculates the power dissipation.

## Use cases

- Calculating the resistor for a single red status LED running on a 5V microcontroller pin.
- Configuring a series array of three white LEDs powered by a 12V automotive battery.
- Verifying power dissipation to choose between 1/4 watt and 1/2 watt resistors for a custom PCB.

## Frequently asked questions

### Why does an LED need a series resistor?

LEDs cannot limit their own current. Without a series resistor, they will draw excessive current and burn out.

### How do I choose between E12 and E24 resistor series?

Choose E12 for standard 10% tolerance resistors, or E24 for more precise 5% tolerance resistors commonly found in modern kits.

### What happens if my supply voltage is lower than the total LED forward voltage?

The LEDs will not turn on because the supply voltage must exceed the combined forward voltage drop of all series LEDs.

### How do I determine the wattage rating for my resistor?

Select a resistor with a power rating at least double the calculated power dissipation to ensure safe operation.

### Can I use this calculator for LEDs connected in parallel?

No, this calculator is designed specifically for single LEDs or multiple LEDs connected in a series configuration.

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