FixFile
File Format

ICO (.ico)

Windows Icon format that stores multiple image sizes in a single file. Used for application icons, website favicons, and Windows desktop shortcuts.

Extension
.ico
MIME Type
image/x-icon

Last updated

Overview

ICO (Windows Icon) is a container format developed by Microsoft that stores one or more images at multiple resolutions and colour depths within a single file. A typical application ICO file contains the same icon at 16×16, 32×32, 48×48, 256×256, and sometimes additional sizes — Windows selects the appropriate size based on display context (taskbar, desktop, file explorer).

ICO is most commonly encountered as the favicon format for websites (favicon.ico at the website root). Every browser requests favicon.ico automatically when loading a website. Modern web development practices supplement ICO favicons with PNG versions at various sizes for different device contexts (Apple Touch icons, Android Chrome icons).

Common Uses

  • Website favicons — favicon.ico is requested by every browser; the standard format for browser tab icons
  • Windows application icons — .exe and .dll files embed ICO images for display in file explorer and taskbar
  • Windows desktop shortcuts — shortcut (.lnk) files reference ICO images for custom icons
  • Windows setup installers — software installers use ICO for their setup executable icons

Advantages

  • Multiple sizes in one file — a single ICO can contain every resolution Windows needs without multiple requests
  • PNG compression — modern ICO files can contain PNG-compressed 256×256 images for high DPI screens
  • 32-bit alpha — supports full transparency (unlike the 1-bit mask transparency of older formats)
  • Universal Windows recognition — every version of Windows since 3.0 renders ICO files natively

Limitations

  • Web use is declining — modern websites use <link rel="icon"> with PNG/SVG files and only keep ICO for legacy browser compatibility
  • Limited non-Windows support — macOS and Linux do not use ICO for native icons (they use ICNS and PNG respectively)
  • Complex multi-size management — creating a properly structured multi-size ICO requires dedicated tools
  • Maximum size limit — the standard ICO format supports images up to 256×256 pixels

Supported Software

  • Creation: IcoFX, RealWorld Icon Editor, GIMP (export as ICO), ImageMagick, online favicon generators
  • Viewing: Windows Explorer (native), IrfanView, XnView
  • Web tools: realfavicongenerator.net, favicon.io

Tools for ICO files