About ICO (Windows Icon)
ICO is the icon container format Microsoft built for Windows, and it doubles as the classic favicon format on the web — browsers have been requesting favicon.ico since the 1990s. A single .ico file can hold several versions of the same image at different sizes, such as 16×16, 32×32, 48×48, and 256×256, so the operating system or browser can pick the sharpest one for each context.
You would create an ICO for two jobs: a website favicon, or an icon for a Windows application, folder, or shortcut. Modern browsers also accept PNG and SVG favicons, but ICO remains the one format every browser and every Windows version understands.
Convertmaxxing converts any supported image to ICO entirely in your browser, for free, with nothing uploaded. One thing to know before you start: the format tops out at 256×256 pixels, so larger images are automatically scaled down to fit.
Strengths
- The most universally compatible favicon format — recognized by every browser, old and new
- Full alpha transparency, so icons composite cleanly over any background
- Can bundle multiple resolutions in a single file
- Native to Windows for app, folder, and shortcut icons
Limitations
- Hard 256×256 size limit — anything larger gets resized down during conversion
- Only useful for icons; it is not a general-purpose image format
- Many image editors cannot open or save ICO without a plugin