What Is an ICO File? The Icon and Favicon Format Explained

Why an icon file holds multiple sizes

An icon has to look sharp in wildly different contexts: 16×16 pixels in a browser tab, 32×32 in a taskbar, 48×48 in a file listing, and larger still on a high-resolution display. Scaling one image down to all of those produces muddy results at the smallest sizes, where every pixel counts. The ICO format solves this by packing several purpose-made versions of the icon into one file — a crisp, hand-tuned 16-pixel version, a 32-pixel version, and so on — and letting whatever displays it choose the best match.

Each embedded image supports transparency, so an icon can be a non-rectangular shape that sits cleanly on any background — essential for logos and app marks that are not simple squares.

ICO and favicons on the modern web

For a long time, every website's favicon lived in a file literally named favicon.ico at the site root, and browsers looked for it automatically. That still works and remains the most universally compatible approach, which is why favicon.ico persists. Modern sites usually supplement it with PNG icons in various sizes (declared in the page's HTML) and an SVG icon for crispness, but a classic .ico is still the safe baseline that every browser, old and new, understands.

For Windows application icons and shortcuts, ICO remains the native format the operating system expects. So while the wider world has moved to PNG and SVG for most graphics, ICO holds on in these two specific niches: Windows icons and the fallback favicon.

ICO vs PNG and SVG

A single PNG is simpler and more universally editable, and modern browsers happily accept PNG favicons — but a PNG holds only one size, so you would supply several separate files to cover every context, where one ICO bundles them. SVG, being vector, scales to any size from one file and is increasingly used for favicons, but it is not supported for Windows application icons and not by the oldest browsers.

The practical takeaway: use ICO for a Windows app icon or a maximally-compatible favicon.ico, and PNG or SVG for everything else. Many sites simply provide all three so every browser and device finds something it likes.

How to create and convert an ICO

You generally make an ICO by starting from a clean, square source image — usually a PNG with a transparent background — and generating the icon from it. Design your logo or mark at a larger size, keep it simple enough to read at 16 pixels, and convert it to ICO (ideally embedding several sizes). To go the other way and pull a PNG out of an existing icon, or to prepare and resize a source graphic before making an icon, an image converter and resizer handle the job. OfficePad's image tools run in your browser, so your artwork stays on your device.

Frequently asked questions