What a ICO file is
What a .ICO file is, what it does well, what opens one, and how to convert it — with every conversion running in your own browser.
What a ICO file is
An ICO is not one image but a small bundle of them at different sizes, in one file. That is its entire reason to exist: an icon has to look right at sixteen pixels in a browser tab and at two hundred and fifty-six on a desktop, and shrinking one large image to sixteen pixels produces mush.
So the file holds a version drawn or scaled for each size, and whatever displays it picks the one closest to what it needs. A well-made ICO contains 16, 32, and 48-pixel versions at minimum, often more.
It is a Windows format that the web adopted almost by accident. `favicon.ico` at the root of a site has been a convention since 1999, and although modern browsers now accept PNG and SVG favicons, the ICO is still what everything falls back to.
Where it came from
ICO comes from Windows 1.0 in 1985. Internet Explorer 5 introduced the favicon in 1999 by simply asking every site for `/favicon.ico`, and because it asked whether or not the site had one, every web server administrator on earth learned the filename from their error logs.
ICO at a glance
| Property | ICO |
|---|---|
| Compression | Lossless |
| Colour | 8 bits per channel |
| Transparency | Full alpha channel |
| Animation | No |
| Keeps EXIF | No |
| Browser support | Every browser since 1999 |
| Size of a photograph | Not a format for photographs |
| Published | 1985 |
ICO at a glance
- Compression is lossless, so converting between it and another lossless format is a repackaging rather than a re-encoding: nothing is decided and nothing is lost.
- Colour is stored as 8 bits per channel, which is enough for the gradients in a photograph to look smooth rather than stepped.
- Transparency is a full alpha channel: each pixel records how transparent it is, so a soft edge stays soft against any background.
- There is no standard place for camera data, dates, or copyright, so anything the source carried is dropped on the way in.
What it is good at
- Full transparency: each pixel records how transparent it is, so edges blend cleanly into any background.
- Nothing is discarded. What you put in is what comes back, however many times you save it.
- Every browser in use reads it, and has for long enough that it needs no fallback.
Where it falls short
- One frame only. An animation converted to it keeps the first frame and drops the rest.
- No standard place for camera data, dates, or copyright, so those are lost on the way in.
What to use instead of ICO
- For anything that moves, use animated WebP. GIF also animates and is more widely accepted, but stores the same clip at several times the size with a fraction of the colours.
When to use ICO
Choose ICO for a favicon at the root of a website, and for a Windows application or shortcut icon. Those are the two jobs it exists for and it is still the best answer to both, because it holds several sizes at once and lets whatever displays it pick the right one.
Avoid ICO for anything that is not an icon. It is not a general image format: the sizes are small by design, the tooling around it is thin, and an ordinary picture stored as an ICO is a picture in the wrong container.
What opens a ICO file
On a computer, ICO files open in GIMP and dedicated icon editors; most image editors will not write one.
Every browser in use displays them, so opening one is usually a matter of dragging it into a browser window.
Turn a ICO file into something else:
Produce a ICO file from something else:
Common problems with ICO
- The camera data and copyright fields have disappeared.
- The format has no standard place to keep them, so anything the source carried was dropped on the way in. Convert to a format that holds metadata if attribution matters, or keep the original alongside the converted copy as the record of where it came from.
Converting a ICO file here
- Choose your file, or drag it onto the box. It is read from your device and goes no further.
- Set whatever the conversion offers — quality, size, background — and start it. Everything runs on your own machine, so a large image uses your memory and your processor rather than a queue somewhere.
- Download the result. Nothing is kept: leaving the page or resetting it releases the file and the converted copy alike.
Common questions
- What sizes should my favicon contain?
- 16, 32, and 48 pixels covers essentially everything in practice: the tab, the bookmark bar, and the Windows taskbar. Adding 64 and 256 costs a few kilobytes and covers high-density displays and desktop shortcuts. Start from a square image at least 256 pixels across so nothing has to be scaled up.
- Can I just use a PNG as a favicon instead?
- Modern browsers accept one, yes, and a single PNG is simpler to manage. An ICO is still worth having at the site root, because it is what browsers request when nothing tells them otherwise, and because a request that 404s is a request wasted on every page load.
- How do I add a favicon to my site?
- Put the ICO at the root of the site as favicon.ico, which browsers request whether you reference it or not. Add link elements pointing at PNG versions as well if you want higher-resolution icons on modern browsers and mobile home screens.
- Is transparency kept when I convert a ICO file?
- Only if the format you convert to has somewhere to put it. PNG, WebP, AVIF, and GIF all do, to varying degrees; JPG and BMP do not, and a transparent area becomes a solid colour when you convert to one. The tool tells you which background it used.
- How do I open a .ICO file?
- The software listed above reads them directly. If nothing on your machine will, converting to PNG or JPG is the reliable route — both open in everything, and the conversion happens here in your browser rather than on a server.
- Does converting a ICO file lose quality?
- It depends on what you convert it to rather than on what you convert from. Converting to a lossless format such as PNG preserves every pixel; converting to JPG, WebP, or AVIF discards detail permanently. Nothing recovers quality the file had already lost.
- Is converting from ICO free, and does it upload my file?
- Free, with no account, and nothing is uploaded. Every converter on this site reads your file with the browser's own file APIs, converts it using code running on your device, and hands the result back as a download. You can confirm that yourself in the Network panel of your browser's developer tools.
- Where does the conversion happen?
- In your browser, on your own device. There is no conversion server here to upload anything to, and no third-party service is involved. That is a statement about what the site does rather than a promise about how it handles uploads, because no upload takes place.