ICO is the Windows icon format, and the format of favicon.ico. A real favicon is rarely one image: an ICO file is a small directory of images at different sizes, and the browser or operating system picks the one that fits.

ico/favicon-16-32-48.ico contains three images, at 16x16, 32x32 and 48x48 pixels, the shape a real favicon has. Software that reads only the first entry, or assumes an ICO holds a single image, shows the wrong size or fails outright.

Use it to test favicon handling in crawlers and link previewers, icon extraction in file managers, image libraries that must choose an entry, and upload forms that accept icons. Because each entry is a different size, it also checks that a thumbnailer picks the largest image instead of scaling up the smallest. For single-resolution images, see PNG.

Every ICO file in one command, each checked against its hash: npx loremfile get --format ico, or loremfile get --format ico after pip install loremfile.

Sample ICO favicon for testing, free and CC0

Related formats