ICO file format
Windows Icon
ICO is the Windows icon format and the classic favicon.ico. One ICO file holds several square images (16 to 256 px) so each screen can use the sharpest size.
| Property | ICO |
|---|---|
| File extensions | .ico |
| MIME type | image/x-icon |
| Introduced | 1985 by Microsoft |
| Compression | PNG or uncompressed bitmap entries |
| Transparency | Yes |
| Animation | No |
| Colour depth | Up to 32-bit + alpha |
| Support | All browsers (favicons), Windows |
| Best for | Favicons, Windows app and shortcut icons |
What is an ICO file?
ICO is the icon format introduced with Microsoft Windows 1.0 in 1985. Unlike other image formats, an ICO is a container: a short directory followed by one or more images, each a different size or colour depth. Windows picks the best match for each view — 16 × 16 in lists, 32 or 48 in Explorer, up to 256 × 256 on high-DPI desktops.
Classic ICO entries are bitmaps with a separate 1-bit transparency mask. Since Windows Vista, entries can also be full PNG images, which allows smooth alpha transparency and much smaller 256 × 256 icons.
ICO and favicons
Browsers have requested /favicon.ico from websites since Internet Explorer 5 in 1999, and many still do automatically. A good favicon.ico includes at least 16 × 16 and 32 × 32 images, with 48 × 48 for Windows shortcuts. Modern browsers also accept PNG or SVG icons via a link tag, but shipping an ICO keeps older browsers and tools happy and avoids 404 errors in your logs.
When should you use ICO?
Use ICO for your website's favicon.ico, Windows application icons, installers and custom shortcut or folder icons. ICO images are limited to 256 × 256 pixels, so it's not a general-purpose image format.