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Every other image format on your disk holds one picture. ICO holds several, and understanding why explains both what the format is for and why it has survived on the web long after Windows icons stopped being most people’s concern.
An ICO is a container. Inside it sit multiple complete images — typically the same icon drawn at 16, 32 and 48 pixels, sometimes more — each stored separately, each with its own pixel data.
When something needs an icon, it picks the size closest to what it needs from inside the file rather than scaling one image up or down.
That design exists because of a hard limit: automatic downscaling destroys small icons. Shrink a detailed 256-pixel icon to 16 pixels and you get a grey smudge — 16 × 16 is 256 pixels in total, barely enough for one recognisable shape. The only way to have a legible 16-pixel icon is for someone to draw it at 16 pixels, deciding which details to discard and which single stroke carries the meaning.
So ICO is not really an image format. It is a way of shipping a designer’s decisions about several sizes at once.
ICO comes from Windows 1.0 in 1985, which makes its continued presence in web development look like an accident. It is not quite.
Browsers automatically request /favicon.ico from the root of a site, whether or not the page links to anything. That behaviour long predates the modern <link rel="icon"> tag and has never been removed, because removing it would break an enormous number of old sites.
The practical consequence: if you have no favicon at the root, every visitor’s browser still asks for one and gets a 404. It costs almost nothing, but it is a request that never succeeds, forever.
Those three are the classic set and remain the right contents for a favicon ICO. Modern icons sometimes add 64, 128 and 256; inside an ICO, a 256-pixel entry is normally stored as PNG data rather than raw pixels, which is what keeps the file from becoming enormous.
This is the smallest design problem in common use, and it is more extreme than it sounds. You have 256 pixels. Not 256 across — 256 in total.
What survives:
What does not:
The usual mistake is shrinking the company logo. Most logos are drawn to be read at the size of a business card, and at 16 pixels they become an unidentifiable blob. The right move is to design a separate mark: the symbol without the wordmark, or one letter, built specifically for this size.
ICO alone is no longer the whole answer, because sites are saved to phone home screens and displayed in contexts Windows never imagined. A complete set today is usually:
The ICO is the floor, not the ceiling. It handles the request that happens whether you planned for it or not.
Converting a PNG to ICO is the normal route, but there is a distinction worth insisting on. A conversion that takes one image and wraps it produces a single-size ICO — technically valid, and it throws away the format’s only real advantage.
What you want is several images in the file: a 48, a 32, and a 16 that you have looked at and adjusted. If the 16 was produced by automatic downscaling, check it at actual size before accepting it. More often than not it needs simplifying by hand.
Start from a square source with generous margin, since an icon with content touching the edge looks cramped at every size.
Only icons. It is not a general image format: no web page should display an ICO as content, no photograph should be stored in one, and nothing outside the icon use case gains anything from it. For an image you simply want small and sharp, PNG does the job without the multi-size container.
/favicon.ico from the root automatically