ICO format: which sizes an icon needs

What it is

An ICO file is a small directory: a header listing several pictures of the same icon, each at its own size and colour depth, followed by the pictures themselves. Each entry is stored either as a PNG or as a bitmap with a one-bit transparency mask. Windows, browsers and shortcuts read the directory and use the size that suits the place the icon appears.

Cursor files (.cur) use the same layout with a hotspot added. Mac applications use a different format, ICNS, though macOS opens ICO files for viewing.

Where it opens

The rows below are about software a file will actually be handed to, not about a specification. A format is supported when the person you sent it to can open it.

Where	What happens
Browsers, as a favicon	Every major browser reads /favicon.ico, and picks the size it needs from inside it.
Browsers, as an image	Chrome, Edge, Firefox and Safari display an ICO in a page, showing one of its sizes.
Windows	The native icon format for programs, shortcuts and folders; Explorer previews ICO files.
macOS	Preview opens ICO files, but Mac apps use their own icon format (ICNS), not ICO.
Very old software	Windows XP-era programs cannot read PNG-compressed entries, which is why small sizes are written as bitmaps.

Last read against vendor documentation on 31 October 2026. Reviewed quarterly; if that date has slipped, treat the rows above as history rather than fact.

What it costs to write

Written here by this site's own ICO writer: every size is resampled from the original, the 256-pixel entry stored as PNG and the small ones as 32-bit bitmaps with a transparency mask (or PNG for all). An icon with four sizes is written in a fraction of a second. Reading works the other way: the directory is parsed and every entry decoded, bitmap entries by this site's own reader.

Where it goes wrong

An icon with only one large size gets scaled down by whatever shows it, and a 16-pixel tab icon made that way often looks blurred.
A 256-pixel entry stored as a bitmap is about 256 KB on its own; stored as PNG it is usually a few kilobytes.
A picture that is not square has to be padded or cropped; stretched icons look wrong at every size.
Opening an ICO as an ordinary image shows only one of its sizes, which can make an icon look smaller or blurrier than it is.

When this is the right answer

ICO is the right answer for a website’s favicon.ico and for Windows program and shortcut icons. A sensible set is 16 and 32 pixels for browser tabs, 48 for Windows, and 256 for high-density screens and large Windows icons. Modern pages often add a PNG apple-touch icon of 180 pixels alongside.

PNG to ICO on this site packs the sizes you choose into one file, resampled from your original; ICO to PNG lists every picture inside an existing icon and saves any of them.

Questions about ICO

Which sizes should a favicon.ico contain?

16 and 32 pixels for browser tabs and bookmarks, 48 for Windows, and 256 for high-density displays. More sizes make the file larger without helping a website.

Why does one ICO file contain several pictures?

So each place that shows the icon can use a picture drawn for its size, instead of shrinking a large one. A 16-pixel icon drawn from a 256-pixel logo in one step often looks blurred.

Can an ICO be transparent?

Yes. PNG entries and 32-bit bitmap entries carry a full alpha channel; older bitmap entries use a one-bit mask.

How do I get a PNG out of an ICO?

ICO to PNG on this site lists every picture inside the icon with its size and depth; save one, or all of them as PNG.

Is ICO better than PNG for a favicon?

Browsers accept both. ICO's advantage is one file holding several sizes at the address browsers ask for by default, /favicon.ico.

Encoding happens on this machine; the file stays where it is.