What does ICO stand for and what makes it different?
ICO is short for icon, and the file extension marks the Windows icon format. Unlike PNG or JPG, where one file is one image, an ICO is a container. It begins with a small header and a directory that lists every image packed inside, recording each one's width, height, colour count, and the byte offset where its data lives. When Windows or a browser needs an icon, it reads that directory and selects the most suitable entry rather than scaling a single bitmap up or down.
Why does one ICO hold multiple sizes?
An icon shows up in very different places: a 16-pixel favicon in a browser tab, a 32-pixel shortcut on the taskbar, a 48-pixel entry in a file listing, and a 256-pixel tile on the desktop. Rather than scaling one bitmap and risking blur, ICO lets a designer hand-tune each size — simplifying detail at 16x16 and adding richness at 256x256 — and stores them all together. The format also supports several bit depths, from old 16-colour and 256-colour icons up to full 32-bit colour with an 8-bit alpha channel for smooth transparency.
How do PNG-compressed icons work?
Classic ICO entries store raw, uncompressed bitmap data, which makes large icons wasteful. Since Windows Vista in 2007, an individual entry inside an ICO may instead hold a complete PNG image, compressed and with its own alpha channel. This is mostly used for the big 256x256 icons, where PNG compression dramatically shrinks the file. The entry still sits inside the ICO directory like any other; only the pixel data is encoded as PNG. Very old systems that predate Vista cannot read those entries, which is why icon sets often mix PNG-compressed large sizes with plain bitmap small sizes.
Why is the favicon an ICO file?
When Internet Explorer 5 introduced the favicon in 1999, it looked for a file literally named favicon.ico at the site root. The ICO format was a natural fit because the same file could carry the tiny 16x16 tab icon and larger sizes for bookmarks and shortcuts. That convention stuck: browsers still automatically request /favicon.icoeven when you don't link one. Today you can also declare PNG or SVG favicons in your HTML, but a multi-size ICO remains the most universally understood option. To build one from artwork you already have, you can convert a PNG to ICO, and our favicon generator packs the common sizes for you.
PNG vs ICO: which should you use?
For a single image at one size, PNG is simpler, more widely supported, and edits cleanly in any tool. Reach for ICO when you specifically need a Windows icon or a traditional favicon — a file that bundles several sizes so the system can always pick a sharp copy. The two convert freely in both directions: you can convert an ICO back to PNG to extract or edit an entry, then repackage it. If you want guidance on picking the right icon format overall, read the best image format for icons, or browse all of our free conversion tools.
Frequently asked questions
- Can one ICO file contain several sizes?
- Yes. That is the whole point of the format. A single .ico file can bundle 16x16, 32x32, 48x48, and 256x256 versions of an icon, and the operating system picks whichever size fits the place it is being shown.
- Is an ICO file just a renamed PNG?
- No. ICO is a container format with its own header that lists each image inside. Since Windows Vista, those images can be stored as PNG data, but the .ico wrapper, directory, and metadata are still distinct from a plain PNG file.
- Do I still need a favicon.ico today?
- It is no longer strictly required, since modern sites can declare PNG or SVG favicons in HTML. But browsers still request /favicon.ico by default, so shipping one remains the safest way to cover every browser and bookmark.
- What is the largest icon an ICO file can hold?
- The format caps each image at 256x256 pixels. For anything larger you would use a different format such as PNG, since the ICO directory stores width and height in a single byte where 0 means 256.