Icon File (MIME: image/x-icon) is a container format used primarily for website favicons and Windows application icons, capable of holding multiple image sizes and color depths in a single file.
History and Development
ICO was introduced by Microsoft with Windows 1.0 (1985) for desktop application icons. It became the web standard for favicons when Internet Explorer adopted it in 1999. While modern browsers also accept PNG and SVG favicons, ICO remains the most universally compatible format for browser tab icons.
Technical Specifications
- Container: Multi-resolution icon package
- Sizes: Typically 16×16, 32×32, 48×48, 256×256 pixels
- Color depth: 1-bit to 32-bit (including alpha)
- Compression: PNG compression for 256×256 size
- Multiple images: Single .ico file contains multiple sizes
Common Use Cases
ICO is primarily used for website favicons (browser tab icons) and Windows desktop application icons. The multi-size capability means a single file serves different display contexts (tab, bookmark, taskbar, desktop shortcut).
ICO vs Similar Formats
- ICO vs PNG favicon: PNG favicons are simpler but don't package multiple sizes. ICO is more compatible with older browsers.
- ICO vs SVG favicon: SVG favicons scale perfectly and are smaller, but IE and some older browsers don't support them.
How to Open and Edit
Windows displays ICO files natively. Use GIMP, IcoFX, or online tools like favicon.io to create ICO files. For web use, generate from PNG using favicon generators.
Frequently Asked Questions
What sizes should a favicon.ico contain?
At minimum: 16×16 (browser tab) and 32×32 (bookmark/taskbar). Optionally add 48×48 and 256×256 for high-DPI displays.
Can I use PNG instead of ICO for favicons?
Yes, most modern browsers accept PNG favicons via . However, ICO has better legacy compatibility.