ICO file format
ICO is a container that holds several sizes of the same icon in one file, which is exactly what Windows and the favicon.ico every browser requests both need.
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What is ICO?
Microsoft introduced ICO with Windows 1.0 in 1985, and its defining idea has not changed: one file, several images. A single .ico can carry 16×16, 32×32, 48×48 and 256×256 versions, and the system picks whichever fits the context — the tiny one for a title bar, the large one for a desktop preview. Scaling a single big image down never looks as good, because a 16-pixel icon needs shapes redrawn rather than shrunk.
The format has quietly modernised. Early icons were palette-based with a one-bit mask, giving the jagged edges of nineties Windows. Since Windows XP, ICO supports 32-bit colour with a full alpha channel, and since Vista an entry inside the container may be stored as PNG data rather than raw bitmap — which is how a 256×256 icon fits in a sensible number of kilobytes.
On the web, ICO persists because of one convention: browsers request /favicon.ico from the site root even when no link tag mentions it. Modern sites also supply PNG and SVG icons through link tags for higher resolutions and dark-mode variants, but shipping the .ico avoids a stream of 404s in your logs and covers older clients.
| Extension | .ico |
|---|---|
| Category | Image |
| MIME type | image/x-icon |
| Released | 1985 |
| Developed by | Microsoft |
| Compression | Lossless |
| Transparency | Yes |
| Animation | No |
ICO: frequently asked questions
- What sizes should a favicon.ico contain?
- A practical set is 16×16, 32×32 and 48×48; add 64×64 or 256×256 if the icon also serves as an application icon. Those cover browser tabs, bookmarks, the taskbar and Windows shortcuts, and the whole file still stays in the tens of kilobytes.
- Why does my icon look blurry at small sizes?
- Because it was scaled down from one large image instead of being drawn for the size. At 16 pixels there is no room for fine detail — thin lines fall between pixels and turn to mush. Simplify the artwork for the small entries: fewer elements, thicker strokes, stronger contrast.
- Can I just use a PNG as a favicon?
- Yes, modern browsers accept PNG and SVG favicons declared with link tags, and SVG additionally supports dark-mode variants. Keep /favicon.ico as well: browsers request that path automatically, and older clients and some feed readers look for nothing else.
- Does ICO support transparency?
- Yes. Modern ICO files use 32-bit colour with a full alpha channel, so edges are smooth against any background. Very old icons used a one-bit mask where a pixel was either opaque or invisible, which is the source of the hard staircase edges on legacy artwork.
- How large should the ICO file be?
- Usually a few kilobytes to about 100 KB depending on how many sizes it holds. Because it is requested on virtually every page load, keep the entries to the sizes you actually need and let the PNG-compressed entries handle the large ones.
- Why does my new favicon not appear?
- Browsers cache favicons aggressively and often ignore a normal reload. Try a hard refresh, open the icon URL directly to confirm the server serves the new file, and check that no cached copy is being kept by a CDN. It can take a day for the change to show everywhere.