What it is
ICO is the icon format Microsoft introduced with Windows. Unlike ordinary image formats, an ICO file is a small container: it holds several versions of the same icon at different sizes, such as 16 x 16, 32 x 32, 48 x 48 and 256 x 256 pixels. Windows and browsers pick the size that fits the place where the icon appears.
Each image inside an ICO can be stored as a bitmap or, since Windows Vista, as a PNG. PNG storage keeps larger sizes compact and supports smooth transparency.
On the web, ICO is best known as favicon.ico, the small icon in browser tabs. Browsers have looked for this file at the root of a website for a long time, and many still request it automatically.
Strengths
- Several sizes in one file. Each size can be shown sharply instead of being scaled.
- Transparency. Icons can have soft, transparent edges.
- Native on Windows. Program icons, shortcuts and folders use ICO.
- Supported by all browsers as a favicon.
Weaknesses
- Only for icons. ICO is not meant for photos or general images.
- Not widely used elsewhere. macOS, phones and modern web manifests use PNG files instead.
- Few editors create it. Many image programs cannot save ICO directly.
When to use it
Use ICO for Windows program and shortcut icons, and as favicon.ico on your website alongside PNG icons. For iPhone home screen icons and Android or web app icons, a website needs PNG files in specific sizes as well. The favicon guide lists the full set.
How to create an ICO file
- PNG, JPG, SVG or other images to ICO: the PNG to ICO converter creates one ICO file with the sizes you tick: 16, 24, 32, 48, 64, 128 and 256 pixels (16, 32, 48 and 256 by default). Non-square images can be fitted onto a transparent square or cropped from the center. A preview shows every size. Each size is stored as PNG inside the ICO, which Windows Vista and later and all browsers support.
- A complete favicon set: the favicon generator creates
favicon.ico(16, 32 and 48 px) together with the PNG icons, Apple touch icon, Android icons, web manifest and the HTML snippet.
Start from a square image of at least 256 x 256 pixels, or an SVG. Both tools run in your browser, so your images are not uploaded. To learn more about the source formats, see the PNG and SVG format pages.
Frequently asked questions
What sizes should an ICO file contain?
For a favicon, 16, 32 and 48 pixels are enough. For a Windows program or shortcut icon, include larger sizes up to 256 pixels so the icon looks sharp in large icon views.
Can I just rename a PNG to .ico?
No. Renaming does not change the file's content, and an ICO has its own structure. Some browsers may still show it, but Windows and other programs can fail. Create a real ICO with the converter.
Do I still need favicon.ico if I have PNG favicons?
Current browsers use the PNG icons linked in your HTML. A favicon.ico at the site root is still a useful fallback, because some browsers and tools request it automatically.