Origin
ICO comes from the desktop icon world. Operating systems needed tiny graphics that could appear at several sizes without forcing every program to guess which image to load. The web later borrowed the format for favicons, and the little /favicon.ico request became part of site culture.
Modern browsers can use PNG and SVG icons too, but ICO remains useful because it lets one file bundle several raster icon sizes and because old software still looks for it.
Technical characteristics
An ICO file is a small container. It can hold multiple images, often 16x16, 32x32, and 48x48 pixels, and modern ICO files may store PNG-compressed image data internally. The browser or operating system chooses the closest size for the context.
The format is not meant for photographs, large artwork, or responsive illustration. Icon design also has a visual constraint: the image must remain recognizable at tiny sizes, which often means simpler shapes, stronger contrast, and fewer details than a normal logo export.
Where it fits
Use ICO when creating a favicon for broad compatibility or a Windows icon resource. Also provide modern PNG and SVG icons in your HTML manifest when possible, because they give browsers more choices and cleaner high-density rendering.
A practical rule: build the icon as SVG or a high-resolution PNG first, then export a multi-size ICO as a compatibility file rather than as your only source asset.
Best uses
- Favicons with legacy compatibility
- Windows desktop icons
- Bundling several small icon sizes
Use another format when
- Large artwork or photos
- Primary brand masters
- Complex icons that collapse at 16x16