Favicon and app icon requirements

One source, several targets

A browser favicon usually needs small raster sizes such as 16, 32, and 48 pixels. Home-screen and platform icons use larger square PNGs, commonly 180, 192, or 512 pixels depending on the target.

An ICO file can contain multiple image frames. Supplying more than one size lets the browser choose a suitable representation instead of scaling a tiny image everywhere.

Check the output

• Use a square source with enough contrast at 16 pixels.
• Keep transparent edges intentional and inspect against both light and dark backgrounds.
• Declare the icon with the correct filename, MIME type, and link relation for the target.

References

• WHATWG — Link type icon
• Apple — App icons