ICO file format (.ico)
Windows / favicon icon. Written as a multi-size PNG icon bundle.
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What is an ICO file?
In short: An ICO file (.ico) is an image file in the ICO format. An ICO file is a bundle rather than a single image: it holds the same icon at several sizes, so the system can pick the right one for a taskbar, a title bar or a desktop shortcut without scaling anything. It can be converted to JPG, PNG or WEBP for free in a web browser with iHateFiles, without uploading it.
The Windows icon format, present since Windows 1.0 in 1985 and later adopted by browsers for favicons.
An ICO file is a bundle rather than a single image: it holds the same icon at several sizes, so the system can pick the right one for a taskbar, a title bar or a desktop shortcut without scaling anything. Modern ICO files store each size as an embedded PNG.
What ICO is good at
- Several sizes in one file, so nothing is scaled badly
- The one format every browser accepts at /favicon.ico
- Transparency throughout
Where ICO falls short
- Only useful at icon sizes
- Not a general image format
- Very old readers only understand a handful of specific sizes
When to use ICO
Use ICO for a website favicon and for Windows application icons. Include 16, 32 and 48 pixel versions at minimum.
How to open a ICO file
Windows and every browser. Image editors vary in how well they write one.
| Windows | Windows shows ICO files as icons rather than opening them. IrfanView or GIMP will show the individual sizes inside. |
|---|---|
| macOS | Preview opens it and shows the largest size only. |
| Linux | GIMP reads and writes every size in the file. |
| Phone | Not practical without a dedicated app. |
| In a browser | Browsers display ICO — that is what a favicon is. |
The thing that usually goes wrong
Renaming a PNG to favicon.ico works in most browsers and then fails in the one that matters. A real ICO bundle avoids the argument entirely.
Questions
What is a ICO file?
Windows / favicon icon. Written as a multi-size PNG icon bundle. Files use the .ico extension.
How do I open a ICO file?
Most image software opens ICO directly. If yours does not, convert it here to something more common. It takes a few seconds and nothing is uploaded.
Can I convert ICO for free?
Yes. Every conversion here is free and unlimited, because your own device does the work rather than a server we have to pay for.
Is ICO lossless?
Yes. ICO stores every pixel exactly, at the cost of a larger file.