ICO File Format (Windows Icon Format)
ICO is the icon file format used by Microsoft Windows for application icons, shortcut icons, and most importantly, website favicons. An ICO file is unique in that it can contain multiple images at different sizes and color depths within a single file, allowing the operating system to select the most appropriate version for each display context. Standard favicon sizes include 16x16, 32x32, and 48x48 pixels. While modern web practices also support PNG and SVG favicons, ICO remains the most universally compatible favicon format and is required for legacy browser support. ICO files can contain either BMP or PNG data for each image layer.
Quick Facts
- Extension: .ico
- MIME Type: image/x-icon
- Category: image
Advantages
- Universal favicon format for all browsers
- Can contain multiple sizes in one file
- Required for legacy browser compatibility
- Native Windows desktop icon format
- Supports transparency for clean icon display
Disadvantages
- Limited to icon-sized images (typically up to 256x256)
- Outdated for general image use
- Larger than single PNG favicons
- Complex multi-image structure
- Not suitable for non-icon purposes
Common Use Cases
- Website favicons (favicon.ico)
- Windows desktop application icons
- Windows shortcut and system icons
- Browser tab and bookmark icons
- Windows taskbar and Start menu icons
Technical Details
ICO files start with a 6-byte header specifying the image type (1 for icons, 2 for cursors) and the number of images. Each image entry has a 16-byte directory entry describing width, height, color count, and data offset. Individual images within the ICO can be stored as BMP data (without the BMP file header) or as full PNG files (for 256x256 and larger). Windows Vista introduced PNG-compressed icons. Standard favicon sizes are 16x16 (browser tab), 32x32 (shortcut), and 48x48 (Windows Explorer). Apple uses apple-touch-icon.png instead of ICO.
Frequently Asked Questions about ICO
What size should my favicon.ico be?
Include at least 16x16 and 32x32 pixel versions. For best Windows support, also include 48x48. Multi-size ICO files are recommended for broad compatibility.
Can I use a PNG as a favicon instead of ICO?
Yes, modern browsers support PNG favicons via <link rel="icon" type="image/png">. However, ICO is still recommended for maximum compatibility, especially with older browsers.
How do I create an ICO file?
Convert a PNG image to ICO using FileChange. The converter creates a properly formatted multi-size ICO file from your source image.
Do I still need favicon.ico in 2024?
Yes. Many browsers still request /favicon.ico by default, and it remains the safest format for universal browser support.