Image format
What is a .ico file?
ICO is the Windows icon format, also used for website favicons. One file can hold the same icon at several sizes, and the system picks the right one.
Each image inside is stored as a bitmap or a PNG, with transparency. Sizes go up to 256 × 256 pixels.
fileclay opens and writes ICO
ICO vs PNG vs SVG
| ICO | PNG | SVG | |
|---|---|---|---|
| Sizes per file | Several | One | Any, it scales |
| Transparency | Yes | Yes | Yes |
| Windows icons | Yes | No | No |
| Favicons | Yes | Yes | Modern browsers |
| Best for | Windows apps, favicons | General graphics | Logos, modern favicons |
When to use ICO
Good for
- Website favicons
- Windows application icons
- Shortcuts and folders on Windows
Not a good fit
- Images larger than 256 px
- Photos
- General use: PNG opens in more apps
How to open .ico files
| Platform | App | Notes |
|---|---|---|
| Windows | Photos, File Explorer | Built in |
| macOS | Preview | Built in |
| Browsers | Chrome, Safari, Firefox, Edge | Drag the file into a tab |
| Editors | GIMP | Shows each size as a layer |
Convert ICO files
From ICO
All 23 output formats: .png, .jpg, .webp, .bmp, .tif, .tiff, .gif, .tga, .jp2, .j2k, .j2c, .ppm, .pgm, .pbm, .pam, .qoi, .pcx, .exr, .sgi, .xbm, .jpeg, .pdf, .pptx
Conversions run in your browser. Files aren’t uploaded.
How to identify a .ico file
ICO files start with two zero bytes, then 01 00, which marks them as icons. The next two bytes say how many images are inside.
It’s read here, in your browser. Only the first bytes.
Your file
What comes after
Questions about ICO
What size should a favicon be?
An ICO with 16, 32 and 48 px covers the usual cases.
Can I rename a PNG to .ico?
Some browsers tolerate it, but it isn’t a real ICO and Windows won’t use it.
Does ICO support transparency?
Yes.
Do I still need a favicon.ico?
Modern browsers accept PNG and SVG, but many sites keep it for older clients.