ICO (icon file)
Also known as: .ico file, icon file format, windows icon
ICO is the Windows icon file format. A single .ico file can hold the same icon at several sizes (such as 16×16, 32×32, 48×48, and 256×256), letting the system pick the right resolution for each context. It is also the classic format for website favicons.
- Holds one icon at multiple sizes in a single file
- Standard format for Windows icons and favicon.ico
- Files are small — usually kilobytes
What makes ICO different
Most image formats store one picture at one resolution. ICO is a container that bundles multiple sizes of the same icon, often with transparency, so Windows can show a crisp icon whether it is a tiny taskbar button or a large file thumbnail.
Icons inside an ICO are typically stored as PNG or uncompressed bitmap data. Because the format targets small dimensions, individual ICO files are tiny — usually a few kilobytes to a few hundred kilobytes.
ICO and favicons
For years, browsers expected a site’s favicon at favicon.ico, and that file still works everywhere as a reliable fallback. Modern sites often add PNG or SVG icons alongside it for higher-resolution displays.
To make one, convert a square PNG to ICO and include several sizes in the same file so it stays sharp at every scale.