About ICO (Windows Icon)
ICO is the icon container format Microsoft built for Windows, and it doubles as the classic favicon format on the web — browsers have been requesting favicon.ico since the 1990s. A single .ico file can hold several versions of the same image at different sizes, such as 16×16, 32×32, 48×48, and 256×256, so the operating system or browser can pick the sharpest one for each context.
You would create an ICO for two jobs: a website favicon, or an icon for a Windows application, folder, or shortcut. Modern browsers also accept PNG and SVG favicons, but ICO remains the one format every browser and every Windows version understands.
Convertmaxxing converts any supported image to ICO entirely in your browser, for free, with nothing uploaded. One thing to know before you start: the format tops out at 256×256 pixels, so larger images are automatically scaled down to fit.
Strengths
- The most universally compatible favicon format — recognized by every browser, old and new
- Full alpha transparency, so icons composite cleanly over any background
- Can bundle multiple resolutions in a single file
- Native to Windows for app, folder, and shortcut icons
Limitations
- Hard 256×256 size limit — anything larger gets resized down during conversion
- Only useful for icons; it is not a general-purpose image format
- Many image editors cannot open or save ICO without a plugin
Designing something that still reads at 16 pixels
The hardest constraint in icon work is not technical, it is visual. A favicon is often displayed at 16×16 — roughly the size of a lowercase letter in this sentence — and almost nothing survives that reduction intact. Detailed logos turn to mush, thin strokes disappear entirely or alias into grey smears, and multi-word wordmarks become an unreadable texture. Scaling down a complex logo and hoping for the best is the single most common mistake.
What works at that size is brutal simplification: one bold shape, one or two high-contrast colours, and no fine detail whatsoever. Many recognisable brands use a completely different mark for their favicon than for their main logo — often a single letter or an isolated element from the larger design. If you are making a favicon from an existing logo, it is usually worth cropping to the strongest element rather than shrinking the whole thing.
Favicons in practice, beyond favicon.ico
The original convention was simple: put a file called favicon.ico at the root of your site and browsers would find it without being told. That still works everywhere, which is exactly why ICO remains worth generating even though modern browsers happily accept PNG and SVG favicons declared with a link tag. It is the reliable baseline that needs no markup and never fails.
Modern sites typically layer on top of it rather than replacing it: an ICO at the root as a fallback, a larger PNG or an SVG for browsers that prefer one, and separate larger images for mobile home-screen icons. Because ICO can hold several sizes in one file, a single well-made ICO covers the browser tab, the bookmarks bar, and the desktop shortcut without any of them looking soft.
Frequently asked questions
- What size should my favicon be?
- Start from a square source image of at least 256×256 and let the conversion handle the rest. The sizes that actually get used are 16×16 in browser tabs, 32×32 on high-density displays and in bookmarks, and 48×48 or larger for desktop shortcuts. Because ICO can carry multiple resolutions in one file, you do not have to pick just one.
- Why does my ICO look blurry in the browser tab?
- Usually because the source image had too much detail to survive being scaled to 16 pixels, or because it was not square to begin with and got squashed. The fix is design rather than technical: crop to the single strongest element of your logo, drop any text, and increase contrast before converting.
- Do I still need an ICO if browsers accept PNG favicons?
- Not strictly, but it remains the most reliable option. A favicon.ico at your site root is found automatically by every browser ever made, with no link tag required, and it never fails on older software. Most sites use it as the baseline and declare a PNG or SVG alongside it for browsers that prefer those.
- Can I convert an ICO back to a PNG?
- Yes, and it is a common task when you want to edit an existing icon. Bear in mind an ICO may contain several sizes; the conversion produces a standard image you can work with, but it cannot recover detail beyond the largest resolution stored inside the file.