What is a .ico file?

About the format

An .ico file is a Windows icon: a container holding the SAME image at several sizes and colour depths (16×16 up to 256×256), so the system can pick the right rendition for a desktop shortcut, an Explorer list, the taskbar or a browser tab — favicon.ico being the web's borrowing of the format. That multi-resolution structure is why you can't just rename a PNG to .ico and expect crisp results: proper icons are exported by an editor (GIMP and many online converters do it well) with each size generated deliberately. Everyday encounters with .ico are mostly customisation — assigning a custom icon to a folder or shortcut via Properties → Change Icon — and the occasional curiosity of finding icons embedded inside .exe and .dll files (Change Icon can browse into those too; shell32.dll is the classic treasure chest). Icons are image data, not code, and carry no meaningful risk; the only 'icon danger' is the old trick of giving an executable a document's icon, which is about the .exe, not the .ico.

How to open a .ico file

Is a .ico file dangerous?

No — it's image data and carries no meaningful risk.

Related file types