What is a ICO file?

ICO — Windows icon

What is it?

ICO is the Windows icon format. Its distinctive feature is that it bundles several versions of the same image inside a single file: 16, 32, 48, 256 pixels square, in various colour depths.

The system then automatically picks the size that fits the context — a thumbnail in a list, a large icon on the desktop — without ever having to resize anything.

History & origin

The format dates back to the earliest versions of Windows in the 1980s and evolved alongside them: transparency was added, then internal PNG compression for the larger sizes.

The web gave it a second life through the favicon, that small icon shown in the browser tab, long served under the name favicon.ico.

Why it is useful

Multiple resolutions in a single file: exactly what an icon needs, since it must stay legible at both 16 pixels and 256.

It remains universally recognised by browsers, including older ones.

Limits & drawbacks

It's a single-purpose format: it's only good for icons, nothing else.

It's poorly suited to very high-density screens, where SVG and large PNGs have now taken over.

Practical tips

For a favicon, start from a decent-sized square PNG and convert it to ICO: the multiple sizes will be generated automatically.

Also provide a 512-pixel PNG and, if possible, an SVG: modern browsers and systems prefer them, and you'll be covered everywhere.

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