Key facts
- MIME type image/vnd.microsoft.icon (image/x-icon is widely used too).
- Holds multiple images, commonly 16x16, 32x32 and 48x48 for favicons and up to 256x256 for Windows app icons.
- Each image is stored as a BMP-style bitmap or, for larger sizes, an embedded PNG.
- A favicon.ico is typically 1-15 KB; browsers request /favicon.ico from the site root by default.
- Related .cur files use the same structure for mouse cursors.
How to open a .ico file
Windows
File Explorer shows ICO thumbnails and Photos opens them. Edit with GIMP or Paint.NET (with an ICO plugin).
macOS
Preview opens .ico files and lists each embedded size in the sidebar. GIMP can open and export ICO as layers.
Linux
Open in GIMP or an image viewer. List and extract sizes with icotool -l favicon.ico and icotool -x favicon.ico (package icoutils), or build one with magick logo.png -define icon:auto-resize=16,32,48 favicon.ico.
Common problems and fixes
- Favicon does not appear or still shows the old icon
- Browsers cache favicons aggressively. Confirm /favicon.ico returns 200 with an image type, add a <link rel="icon"> tag, and hard-refresh or append a version query (favicon.ico?v=2).
- Icon looks blurry in tabs or on the desktop
- The file lacks the exact size being displayed, so it is scaled. Include 16, 32 and 48 px images (and 256 px for Windows apps) drawn at those sizes.
- Renamed PNG to .ico does not work everywhere
- Modern browsers often accept a PNG renamed to .ico, but Windows and some tools need a real ICO container. Generate a proper multi-size ICO instead.
- favicon.ico requests return 404 in server logs
- Browsers request /favicon.ico even if your HTML points elsewhere. Place a favicon.ico in the web root to stop the errors.
Often converted to or from: PNG, SVG, JPG, ICNS (macOS icon)