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ICO Format — Complete Guide & Converter

Everything you need to know about the ICO file format: technical specifications, use cases, compatibility, and free conversion tools.

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What is a ICO File?

ICO is the icon file format of Microsoft Windows. Unlike ordinary image formats, an ICO file is a container: it holds several images of the same icon at different sizes and color depths, such as 16 × 16, 32 × 32, 48 × 48 and 256 × 256 pixels, and the operating system or browser picks the most appropriate one for where the icon is shown. Each image in an ICO can be stored as a bitmap (with an AND mask for transparency, or 32-bit color with a full alpha channel) or, since Windows Vista, as a complete PNG image, which keeps large icons compact. The format's header stores the width and height of each entry in a single byte, which is why 256 × 256 pixels is the largest standard icon size. On the web, ICO is best known as the format of favicon.ico, the small icon browsers show in tabs, bookmarks and history.

History of the ICO Format

The ICO format dates back to Windows 1.0 in 1985, when icons were tiny monochrome images. Windows 3.0 introduced 16-color icons, and Windows 95 brought 256-color icons and the familiar 16 × 16 and 32 × 32 sizes for the Start menu and desktop. Windows XP (2001) added 32-bit icons with an 8-bit alpha channel for smooth, anti-aliased edges. Windows Vista (2007) introduced 256 × 256 icons and allowed them to be stored as PNG data inside the ICO file to reduce size. On the web, Internet Explorer 5 introduced the favicon in 1999 by requesting /favicon.ico from each site, a convention every browser still follows. Modern websites often supplement favicon.ico with PNG and SVG icons and web app manifests, but the multi-size ICO remains the most widely supported favicon and the required format for Windows application icons.

ICO Technical Specifications

File Extension.ico
MIME Typeimage/vnd.microsoft.icon (also image/x-icon)
StructureContainer of multiple icon images
Typical Sizes16, 32, 48, 64, 128, 256 px
Maximum Size256 × 256 pixels
TransparencyFull alpha (32-bit) or 1-bit mask
Image DataBMP-style bitmaps or embedded PNG
Developed ByMicrosoft (Windows 1.0, 1985)

ICO Advantages & Limitations

Advantages

  • ✓Holds multiple sizes of an icon in one file
  • ✓Windows and browsers automatically choose the best size
  • ✓Full alpha transparency (32-bit)
  • ✓Can store large sizes as compressed PNG internally
  • ✓Universal favicon support in every browser
  • ✓Required format for Windows application and shortcut icons

Limitations

  • ✗Maximum size of 256 × 256 pixels
  • ✗Poorly supported by general image editors and design tools
  • ✗Not suitable for general images or photos
  • ✗Multiple sizes must be prepared for best results
  • ✗Small sizes (16 × 16) can't show fine detail

Common Use Cases for ICO

ICO files are used for favicons (a multi-size favicon.ico in a website's root folder is picked up automatically by browsers) and for Windows desktop software: application icons for .exe files and installers, shortcut and folder icons, and icons in Electron, .NET, Qt and other Windows builds. Developers and designers also encounter ICO files when customizing Windows (folder and drive icons), packaging browser extensions or intranet tools, and working with legacy web projects. Because most editors don't handle ICO well, the typical workflow is to design an icon as a large PNG and convert PNG to ICO to generate all the standard sizes, or to convert ICO to PNG to edit or reuse existing icon artwork.

Convert ICO to Other Formats

Transform your ICO files to any of the supported target formats below. Each conversion preserves maximum quality and is completely free.

ICO to PNG

Convert Other Formats to ICO

Need to create ICO files from other formats? Use any of the converters below.

PNG to ICO

ICO vs Related Formats

ICO is a specialized container for icons rather than a general image format. PNG is the usual source and export format for icon artwork, SVG favicons scale to any size on modern browsers, and Apple's ICNS plays the same role on macOS that ICO plays on Windows.

JPG Format Guide PNG Format Guide WEBP Format Guide HEIC Format Guide GIF Format Guide AVIF Format Guide BMP Format Guide TIFF Format Guide JFIF Format Guide

ICO — Frequently Asked Questions

What is an ICO file?

A Windows icon file that contains one icon at several sizes, so the system or browser can display the sharpest version wherever it's shown.

What sizes should a favicon.ico include?

At least 16, 32 and 48 pixels. Including 64, 128 and 256 pixels covers high-density screens and Windows shortcuts too.

How do I open an ICO file?

Windows Photos and File Explorer preview ICO files, browsers display them, and GIMP opens every size as a separate layer. Many other editors don't, so converting to PNG is often easiest.

Do I still need favicon.ico?

It's recommended. Browsers request /favicon.ico by default, and some tools only read ICO, so it's worth providing alongside PNG or SVG icons.

What's the largest ICO size?

256 × 256 pixels, because the format stores each dimension in a single byte.

Can I make an ICO from a PNG?

Yes. PNG to ICO creates a multi-size icon (16 to 256 pixels) from a single PNG image.

Explore Other Format Guides

HEICJPGMP4MP3PNGWEBPWAVAVIAVIFBMPWordFLACGIFJFIFM4AMKVMOVOGGPDFPowerPointTIFFTXTWEBM