What Is ICO? The Icon Format for Windows and Favicons

What ICO is in plain English

ICO is a file format for icons, invented by Microsoft for Windows. It shipped with the very first version of the system, Windows 1.0 in 1985, and ever since it has been responsible for everything that looks like a small recognisable picture: program icons, shortcuts, folder badges. Later the format gained a second life on the web — as the favicon, that little image in a browser tab.

If you had to describe ICO in one line, it would be this: it is not a single picture, but a box holding several sizes of the same logo. And that is exactly what sets it apart from the familiar PNG and JPG.

A container, not a picture: the core idea

Picture a website logo. Different places need it at different sizes: 16×16 pixels in a browser tab, 32×32 on the taskbar, 48×48 in the file explorer, 256×256 for a large tile or preview. If you only had one PNG, say 256×256, the system would have to shrink it on the fly every time. And automatic shrinking of fine detail always produces mush and dirty edges: at 16×16 a beautiful logo would turn to porridge.

ICO solves this elegantly: a single file stores several ready-made versions, each prepared for its own size. For 16×16 a designer can drop the extra detail and keep only the recognisable silhouette, while for 256×256 they can show every nuance. The system simply picks the right picture from the set — no blur, no loss of sharpness.

What is inside the file: BMP and PNG

The structure of ICO is simple and logical. The file starts with a short header that records how many images are inside and where each of them sits — a kind of table of contents. Then come the pictures themselves, one after another.

The interesting part is what format those pictures are stored in. Historically each image inside an ICO is an uncompressed BMP (more precisely a DIB — device-independent bitmap). No compression: pixel by pixel, as is. For 16×16 and 32×32 that is fine — they are tiny anyway.

But once large 256×256 icons appeared, the uncompressed form became wasteful: a single such picture would weigh hundreds of kilobytes. So starting with Windows Vista, ICO was allowed to hold images in the PNG format with compression. That is how a modern ICO ends up mixed: small sizes as good old BMP, the large 256×256 as a compact PNG.

Transparency: mask and alpha channel

An icon almost always needs a transparent background — otherwise an ugly square appears around it. ICO can do this in two ways.

The old way — a transparency mask. The picture comes with an extra 1-bit layer where each pixel is marked "visible" or "transparent." Crude, but enough for the blocky icons of the 1990s.

The modern way — an alpha channel. As in PNG, every pixel has a transparency value from 0 to 255. This gives the icon smooth edges, and semi-transparent shadows and highlights sit on any background without artifacts. That is why a modern logo in a tab looks clean in both the light and dark browser theme.

Where the format came from

ICO is one of the oldest formats still in active use on the PC. It was born together with the Windows graphical interface and for decades was the only way to give a program an icon. The key milestones:

• 1985 — Windows 1.0. The format appears as part of the system. Icons are tiny, with 16 colors or fewer.
• 1999 — favicons. Internet Explorer 5 introduces the idea: drop a favicon.ico into the site root and the browser shows the picture in the tab and in favorites. That is how ICO reached the web.
• 2007 — Windows Vista. Support for large 256×256 icons and PNG compression inside ICO. Icons become genuinely detailed.

Over forty years the format has barely changed on the outside — and that is its strength. An .ico file made today opens on modern Windows and is understood by any browser.

Where ICO is used today

Despite its age, ICO is everywhere — we just rarely notice it.

• Windows app and shortcut icons. Any program on the desktop or in the Start menu shows its badge straight from an ICO.
• Website favicons. The small picture in the tab, in bookmarks, in history and in search results. Browsers still look for /favicon.ico first.
• Compatibility. When a site needs to support both old and new browsers, ICO remains the universal "fallback" that everyone understands.

These days favicons often add PNG and SVG too — which of them to choose has its own dedicated article. But ICO gives maximum compatibility and is still the standard for the tiny sizes in tabs and bookmarks.

How to create and open ICO

Because ICO is a special container, a regular image editor cannot always assemble it. The fastest path is to convert a ready image. Most often an icon is drawn as a PNG (with a transparent background) and then turned into ICO.

And if you instead need to see what is hidden inside someone else's .ico, the reverse conversion comes in handy — ICO to PNG. The full list of supported conversions is always on the all formats page.