ICO (Windows Icon)
The container format that lets a single file hold every size of a Windows icon at once.
| Full name | Windows Icon |
| Extension | .ico |
| MIME type | image/x-icon |
| Developer | Microsoft |
| Released | 1985 (Windows 1.0) |
| Type | Image container |
| Max image size | 256x256 pixels (PNG-compressed since Vista) |
| Color depths | 1-bit monochrome up to 32-bit RGBA |
What is a ICO file?
ICO is a container image format developed by Microsoft for storing small graphical icons used in the Windows operating system. A single .ico file can hold multiple versions of the same icon at different sizes and color depths. Browsers also rely on ICO files for the favicon shown in tabs and bookmarks.
An ICO file is not just one image. It is a bundle of images stored together in one file, each at a different size or color depth. Windows picks whichever image from that bundle best suits the current display context. Each image inside the file is stored either as a BMP-derived bitmap with an AND mask for transparency or, since Windows Vista, as a full PNG for better compression and alpha channel support.
History
Microsoft introduced the ICO format with Windows 1.0 in 1985. The earliest icons were 32x32 pixels and monochrome. As Windows evolved, the format expanded to support 16 colors, then 256 colors, then full 32-bit RGBA color with alpha transparency starting in Windows XP. Windows Vista extended the format again to allow 256x256 pixel icons stored as compressed PNG data inside the ICO container.
How it works
Every ICO file starts with a 6-byte header that identifies the file type and records how many images are stored inside. After the header comes a directory, with one 16-byte entry per image listing the width, height, bit depth, byte length, and offset of that image's data. The image data that follows is either a Device-Independent Bitmap (DIB) paired with transparency mask bitmaps, or a raw PNG file. The multi-image layout means one ICO file can satisfy every context Windows needs, from a 16x16 taskbar thumbnail to a 256x256 high-resolution display.
What it is used for
- Website favicons displayed in browser tabs, bookmarks, and address bars
- Application and file-type icons shown in Windows Explorer and the taskbar
- Desktop shortcut icons for programs and folders
- System tray notification area icons for background applications
How to open it
Windows can display ICO files natively in Explorer with no extra software. For editing, tools like IcoFX, GIMP (with the ICO plugin), or Adobe Photoshop (with a plugin) let you create and modify the individual images inside an ICO file.
Pros and cons
Strengths
- Stores multiple sizes and color depths in one file, so Windows always has the right version
- Supports full alpha-channel transparency since Windows XP
- Can embed losslessly compressed PNG images inside the container since Windows Vista
- Universally supported by all major browsers for favicons without any configuration
Trade-offs
- Maximum image size is 256x256 pixels, which is too small for high-DPI print use
- The BMP-based storage used for smaller sizes has no compression, making those images larger than necessary
- No support for animation; animated icons require a separate CUR/ANI format
- The image/x-icon MIME type is unofficial; the IANA-registered type is image/vnd.microsoft.icon, causing occasional server configuration confusion
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ICO FAQ
What is the difference between ICO and PNG for a favicon?
An ICO file can bundle 16x16, 32x32, and 48x48 versions in a single download, so older browsers always get the right size. A PNG favicon works in modern browsers but serves only one size. Using ICO gives the widest compatibility with no extra HTML required.
Can an ICO file contain a PNG image?
Yes. Since Windows Vista, ICO files can store images as full PNG data instead of raw BMP bitmaps. This is the standard approach for the 256x256 size because PNG compression keeps the file small.
What is the maximum size for an ICO image?
The ICO format supports images up to 256x256 pixels. This limit comes from the directory entry structure, where width and height are each stored in a single byte (a value of 0 is interpreted as 256).
Why does my ICO file look blurry at large sizes?
Windows scales the largest image it finds inside the ICO file if no exact-size match exists. If the ICO only contains a 16x16 or 32x32 image, Windows stretches it and it looks blurry. The fix is to include a 256x256 PNG image inside the ICO file.