What is ICO?
ICO (Icon) is a Windows image format that stores multiple sizes and color depths of a single icon in one file, primarily used for application icons and website favicons.
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What is ICO?
ICO (Icon) is a Windows image format that stores multiple sizes and color depths of a single icon in one file, primarily used for application icons and website favicons.
Understanding ICO helps you choose the right format for your specific needs and workflow.
How ICO Works
An ICO file is a small container holding a directory followed by one or more image entries, each of which may be encoded either as a BMP-style device-independent bitmap or, since Windows Vista, as embedded PNG data for larger sizes.[4] Bitmap entries reuse the DIB header structures defined by Windows, with an additional AND mask providing 1-bit transparency for legacy bitmap icons.[3] Because multiple resolutions live in one file, the operating system can pick the most appropriate size for a given display context.[4]
Use as a Favicon
On the web, ICO became the de facto favicon format, historically requested automatically by browsers at /favicon.ico and referenced through a <link rel="icon"> element.[2] Its ability to bundle several sizes in a single file is convenient for favicons that must render crisply at 16, 32, and 48 pixels.[1]
Modern Context
While ICO remains broadly supported, modern sites increasingly supply PNG or SVG icons through explicit <link> tags, reserving ICO mainly for backward compatibility with older browsers.[2] The closely related CUR format shares the same structure but adds hotspot coordinates for mouse cursors.[4]
ICO Technical Specifications
ICO vs Other Image Formats
| Feature | ICO | PNG | SVG |
|---|---|---|---|
| Type | Raster (container)[4] | Raster | Vector |
| Multiple sizes | Yes[4] | No | Scalable |
| Transparency | Yes[3] | Yes | Yes |
| Browser support | Favicons[1] | Universal | Universal |
| Standardized by | Microsoft[3] | W3C, ISO | W3C |
| Best for | Icons, favicons | General images | Scalable icons |
ICO bundles several icon sizes in one file for favicons and app icons, while PNG and SVG serve broader image and scalable-graphic needs.
Advantages & Disadvantages
Advantages
A single ICO file can contain 16x16, 32x32, 48x48, and 256x256 versions - Windows picks the right size automatically.
Every browser and operating system supports ICO for favicons without any extra configuration.
Full alpha channel transparency in 32-bit ICO allows icons to look perfect on any background.
Icon sizes are tiny - a full multi-resolution ICO file is typically only 50-200KB.
Disadvantages
ICO cannot store images larger than 256x256 pixels, limiting use to icon purposes only.
Creating proper multi-resolution ICO files requires specialized tools - not supported by standard image editors.
ICO originated as a Windows format; macOS uses ICNS instead, requiring format conversion.
Older ICO variants had limited color palettes; modern 32-bit ICO resolves this but compatibility varies.
Common Use Cases
Here are the most common scenarios where ICO is the right choice:
Website Favicons
The browser tab icon that appears next to your site name - ICO is the most universally compatible format.
Windows Applications
Desktop application icons shown in taskbars, start menus, and file explorers.
Desktop Shortcuts
Custom icons for Windows desktop shortcuts and folder icons.
Browser Bookmarks
Icons shown in bookmark lists and browser history for quick visual identification.
Convert ICO Files Online - Free
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Try Image Converter FreeFrequently Asked Questions
What size should a favicon ICO be?
A good favicon.ico includes at least 16x16 and 32x32 sizes. Modern sites often also include 48x48 for Windows taskbar display.
Can I use PNG instead of ICO for favicons?
Yes, modern browsers support PNG favicons. But ICO ensures compatibility with older browsers and Windows integration.
How do I create an ICO file?
Use online converters, GIMP (with ICO plugin), IcoFX, or Photoshop plugins. Most standard image editors cannot save ICO natively.
Why does my favicon not show up?
Clear browser cache, ensure the file is named favicon.ico in the root directory, and check the link tag in your HTML head section.
What is the difference between ICO and ICNS?
ICO is the Windows icon format; ICNS is the macOS equivalent. Web favicons use ICO for universal compatibility.