.ico File — Windows Icon

ICO is the image file format used for icons in the Microsoft Windows operating system and for favicons (website icons) on the web. An ICO file is a container that can hold multiple images at different sizes and color depths, allowing the operating system to select the most appropriate version based on context — smaller icons for taskbars and larger ones for desktop display. Standard sizes include 16x16, 32x32, 48x48, and 256x256 pixels, with the 256x256 version typically using PNG compression for efficiency. ICO files support 1-bit through 32-bit color depth, with 32-bit images including an 8-bit alpha channel for smooth transparency. For web use, ICO files served as the standard favicon format for decades, typically placed at /favicon.ico in a website's root directory. While modern browsers now support PNG, SVG, and other formats for favicons, ICO remains the most universally compatible option and is still recommended for maximum browser support. Windows uses ICO for executable icons, shortcut icons, and system interface elements.