ICO is the icon format of Microsoft Windows, and the original format of website favicons. Its defining feature is that a single ICO file can contain several versions of the same picture at different sizes and colour depths, so each place that shows the icon can pick the sharpest one.
How ICO works
An ICO file starts with a directory listing the images inside, each with its width, height and colour depth. The images themselves are stored as bitmaps or, for large sizes, as PNG data. The maximum size is 256 × 256 pixels. Windows cursor files (.cur) use the same structure, plus a click point.
Favicons
Browsers request /favicon.ico from every website automatically, and show it in tabs, bookmarks and history. A good favicon.ico contains at least 16, 32 and 48 pixel versions. Modern browsers also accept PNG and SVG favicons declared in the page, but favicon.ico remains the most compatible choice.
What ICO is good at
- Multiple sizes in one file, each drawn crisply
- Transparency for icons on any background
- Universal support in Windows and browsers
Where ICO falls short
- 256-pixel limit, too small for app store or high-resolution touch icons
- Limited editing support in many image editors
- Not used on macOS, iOS or Android, which use PNG-based icons
ICO and other formats
Create a multi-size favicon from a square PNG logo with PNG to ICO, or extract the largest image from an icon with ICO to PNG.