What is an ICO File? Icon Sizes, Favicons, and When to Use ICO
Understand the ICO container format, the sizes it holds, and why it still matters for favicons and desktop icons
Convert NowWhat Is an ICO File?
An ICO file is an image format created by Microsoft in the early days of Windows. Unlike a JPEG or PNG, which stores a single image at a single resolution, an ICO file is a container that can hold multiple images at different sizes and color depths inside one file. When an operating system or browser needs to display an icon, it picks the best-fitting image from the set bundled inside the ICO.
This container approach solves a real problem. A tiny 16-pixel tab icon and a 256-pixel desktop shortcut require very different levels of detail, and scaling a single image up or down rarely produces clean results. By embedding purpose-built versions at 16, 32, 48, 64, 128, and 256 pixels, the ICO format lets each context display a crisp, pixel-perfect icon without any runtime resizing artifacts.
Internally, each image stored in an ICO file can be encoded as either a BMP bitmap or a PNG-compressed frame. Modern ICO files typically use PNG compression for the larger sizes (128 px and above) to keep file sizes manageable, while smaller sizes may remain as uncompressed BMP data for maximum compatibility with legacy systems. The file header tells the consuming application how many images are included and at what dimensions, so the right variant is selected automatically.
ICO vs PNG: When Do You Actually Need ICO?
PNG has become the default image format on the web, so it is fair to ask whether ICO is still relevant. The short answer: yes, but only in specific situations. The table below summarizes the key differences.
| Feature | ICO | PNG |
|---|---|---|
| Multiple sizes in one file | Yes | No (one image per file) |
| Browser favicon support | Universal (all browsers) | Modern browsers only |
| Windows desktop icons | Required | Not supported |
| macOS / Linux app icons | Not used | Preferred |
| Transparency | Yes (alpha channel) | Yes (alpha channel) |
| File size efficiency | Larger (multiple images) | Smaller (single image) |
For favicons, ICO remains the safest choice when you need to support every browser, including older versions of Internet Explorer that do not recognize PNG favicons. A favicon.ico placed at the site root is detected automatically by virtually every browser ever shipped. If you only target modern browsers, a PNG favicon declared via a <link> tag works fine, but the ICO fallback costs almost nothing to add.
For Windows desktop and taskbar icons, ICO is the only accepted format. Executable files (.exe) embed their icons as ICO resources, and shortcut files (.lnk) reference ICO files directly. If you are building or theming Windows applications, ICO is non-negotiable.
On macOS and Linux, ICO is generally not used. macOS relies on ICNS files and PNGs, while most Linux desktop environments accept SVG or PNG. So if your audience is exclusively on those platforms, you can skip ICO entirely.
Common ICO Sizes and What They Are For
Not every size serves the same purpose. The table below maps each standard ICO dimension to its typical use, along with a recommendation on whether to include it.
| Size (px) | Where It Is Used | Recommendation |
|---|---|---|
| 16 × 16 | Browser tabs, address bars, bookmark menus | Always include |
| 32 × 32 | Windows taskbar, newer browser tabs on high-DPI screens | Always include |
| 48 × 48 | Windows desktop icons (standard view), Windows Explorer list view | Include for desktop use |
| 64 × 64 | Some Windows UI contexts, high-DPI taskbar | Optional but useful |
| 128 × 128 | Windows large-icon view, some docks | Include for sharp large icons |
| 256 × 256 | Windows extra-large icon view, high-DPI displays | Include for best quality |
For a favicon-only ICO, the essential sizes are 16 and 32 pixels. Adding 48 pixels covers Windows desktop shortcuts that link to your site. For a deeper dive into which favicon sizes matter most, see our guide on favicon sizes explained.
For Windows application icons, you should include the full range from 16 through 256. The 256-pixel variant is especially important on modern high-DPI monitors, where Windows may render desktop icons at very large sizes. Skipping it means the OS will upscale a smaller image, producing blurry or pixelated results.
How to Create an ICO File from Any Image
Creating an ICO file used to require specialized desktop software, but modern browser-based tools make the process fast and painless. Any2ICO accepts PNG, JPG, WebP, GIF, BMP, AVIF, and SVG source images up to 10 MB each and produces a properly structured, multi-size ICO file in seconds.
Here is a quick workflow:
- Choose your source image. Start with the highest resolution available. A 512-pixel or larger source gives the converter enough detail to produce clean downscaled versions at every target size.
- Pick your sizes. Select the sizes you need: 16, 32, 48, 64, 128, and 256. You can choose any combination.
- Set scaling and background options. Choose between contain (fit the image inside the square, preserving aspect ratio) or cover (fill the square, cropping edges). Pick a transparent or white background for any padding areas.
- Convert and download. Any2ICO bundles the selected sizes into a single ICO file. If you are converting multiple source images, you can download them all at once in a ZIP archive.
The entire conversion happens in your browser using the Canvas API. Files you import from your device are sent through the server for processing but are not stored permanently. If you paste from the clipboard, that data never leaves your machine at all.
For a step-by-step favicon tutorial, check out our guide on how to create a favicon.ico.
Key Takeaways
The ICO format may be decades old, but it remains indispensable for two core use cases: favicons and Windows icons. Its ability to bundle multiple resolutions into a single file ensures that every display context, from a 16-pixel browser tab to a 256-pixel desktop shortcut, gets a sharp, purpose-built image.
When you are ready to create your own ICO, remember these principles: start with a high-resolution source, include at least 16, 32, and 256 pixels, and use a tool like Any2ICO that gives you precise control over which sizes and scaling options to apply. For more on choosing the right sizes for your favicon, read Favicon Sizes Explained.