What is an ICO file?
ICO is the Windows icon format, dating back to Windows 1.0. Unlike a normal image it is a container: one file holds the same picture at several sizes and color depths, so Windows can pick the 16-pixel version for a list view and the 256-pixel version for the desktop. Browsers adopted the same format for favicon.ico, the little image in the tab and bookmarks bar.
Which sizes does a favicon need?
16 × 16 for the tab and address bar, 32 × 32 for high-density screens and taskbar pins, and 48 × 48 for Windows shortcuts. Adding 256 × 256 covers app-style icons. Modern ICO files store each size as a PNG, which keeps transparency and file sizes small, and every browser and Windows since Vista accept that.
Making and unpacking icons
Start with a square PNG at least 256 pixels wide, ideally simple and bold since it must read at 16 pixels. The PNG to ICO tool scales it to each size you tick and bundles them. Going the other way, the ICO to PNG tool pulls out the largest image inside so you can edit it.
Frequently asked questions
Can I use a PNG as a favicon instead of ICO?
Modern browsers accept PNG and SVG favicons via a link tag. ICO remains the fallback that everything, including old browsers and Windows, understands.
Why does my icon look blurry at small sizes?
Because a detailed image was shrunk to 16 pixels. Design a simplified version for the small sizes, or start from a bold, high-contrast shape.