What Is ICO Format? The Complete Guide to Favicon and Icon Files
The ICO format is the file type Windows has used for icons and favicons since the very first version of the operating system. If you've ever added a favicon to a website or noticed the small image in a browser tab, there's a good chance an ICO file was involved. This guide explains what ICO files are, how their unusual multi-size structure works, and when you should choose ICO over PNG.
What is an ICO file?
An ICO file (extension .ico) is a container that stores one or more small images — typically icons — at multiple sizes and color depths inside a single file. It was created by Microsoft for Windows and is still the format browsers expect at the classic /favicon.ico path. Unlike a JPG or PNG, which holds a single bitmap, an ICO can bundle a 16×16, 32×32, and 48×48 version of the same icon together so the system can pick the sharpest one for each context.
How multi-size icons work
The strength of ICO is that it's a multi-resolution format. A single file can contain:
- 16×16 — browser tabs and address bars
- 32×32 — taskbar and desktop shortcuts
- 48×48 — Windows Explorer in medium-icon view
- 256×256 — large/extra-large icon views (PNG-compressed inside the ICO)
When an application needs an icon, it reads the ICO and selects the embedded image closest to the size it needs, avoiding the blur you get from scaling one bitmap up or down. Modern ICO files store large sizes (256×256) using PNG compression internally to keep the file small.
ICO vs PNG for favicons
For years, favicon.ico was the only reliable way to give a site an icon. Today browsers also accept PNG and SVG favicons declared with a <link> tag, so which should you use?
| Use case | Best choice | Why |
|---|---|---|
| Maximum compatibility | ICO | Every browser, including very old ones, requests /favicon.ico by default |
| Crisp multi-size favicon | ICO | One file holds 16/32/48 px so each context stays sharp |
| Modern, scalable icon | SVG | Resolution-independent, tiny, themeable — but no legacy support |
| Single high-res app icon | PNG | Simpler when you only need one large size (e.g. apple-touch-icon) |
The practical recommendation: ship a multi-size favicon.ico for compatibility and add PNG/SVG declarations for modern browsers. You get the best of both.
How to create an ICO file free
You don't need a desktop icon editor. Start from a square PNG (512×512 works well) and convert it:
- Open the Image Converter and upload your square PNG
- Choose ICO as the output format
- Download the ICO — it bundles the standard favicon sizes for you
- Place the file at your site root as
favicon.ico
If your source isn't square, crop it first with the Image Cropper so the icon isn't distorted, and use the Image Resizer to get a clean square master before converting.
When not to use ICO
- Photographs — ICO is for flat icons, not photos. Use JPG or WebP instead
- Large in-page graphics — PNG or SVG are far more efficient above icon sizes
- Animated icons — ICO has no animation; use an animated PNG/GIF or SVG
Key takeaways
- ICO is a Windows container that holds an icon at several sizes in one file
- It remains the most compatible favicon format and the default browser request
- Pair an ICO favicon with PNG/SVG declarations for modern browsers
- Create one free with the Image Converter from a square PNG