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File Format Guide
What is an ICO file?
ICO is not really an image format — it is a container that holds several images at once. That design comes straight from Windows 1.0, and it is the reason ICO is still the safe answer for a favicon four decades later.
ICO at a glance
What is an ICO file?
An image file format specifically for computer icons in Microsoft Windows. A single ICO file usually contains multiple sizes (e.g., 16x16, 32x32) of the same image.
How to open an ICO file?
Windows Explorer, web browsers, and image editors (sometimes requiring plugins).
Typical use of an ICO file:
Windows desktop application icons and traditional website "favicons" (the small icon in a browser tab).
Origin of the ICO file:
Introduced by Microsoft with Windows 1.0.
Modern alternatives for ICO:
For web favicons, PNG and SVG are the modern standards. For Apple apps, the ICNS format is used.
ICO specifications
Full name
Windows Icon
File extensions
.ico
MIME type
image/vnd.microsoft.icon (also seen as image/x-icon)
Type
Container holding multiple images at different sizes
Internal data
BMP-style bitmaps, or PNG data (Windows Vista onwards)
Transparency
Supported — 1-bit mask in classic icons, full alpha in 32-bit icons
Typical sizes
16×16, 32×32, 48×48, 256×256
Animation
Not supported (the separate .ani format handles that)
Introduced
1985, with Windows 1.0
Browser support
Universal for favicons
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One file, several pictures
An ICO file begins with a small directory listing each image it contains — width, height, colour depth, and where in the file to find the data. The operating system picks whichever entry best matches the size it needs to draw. That is the whole design, and it exists because scaling an icon down algorithmically produces mush.
At 16×16 pixels there is no room for detail; a legible icon at that size is a different drawing from the same icon at 256×256, usually with thicker strokes and fewer elements. Bundling both means the system never has to guess.
PNG inside ICO
Originally the images inside an ICO were BMP-style bitmaps with a separate one-bit transparency mask, which is why very old icons have hard, jagged edges. Windows XP added 32-bit entries with a proper alpha channel, and Windows Vista allowed entries to be stored as PNG data instead — which matters at 256×256, where an uncompressed bitmap would be a megabyte on its own.
A modern ICO is therefore usually a small mixed bag: raw bitmaps at the tiny sizes, PNG at the large one. Any competent converter handles this for you, but it explains why two ICO files of the same nominal size can differ enormously in bytes.
What a favicon actually needs today
Browsers will happily use a PNG or SVG favicon, and an SVG favicon has the pleasant property of adapting to light and dark browser themes. But a file named favicon.ico at the site root is still fetched automatically by browsers and crawlers that ignore your markup entirely, and it remains the only thing guaranteed to be found without configuration.
The pragmatic setup is both: an ICO at the root containing 16×16, 32×32, and 48×48 for the address bar and bookmarks, plus a larger PNG referenced in your markup for the app icons that phones and desktops use when a site is pinned.
Convert ICO files in your browser
Zolvo's image converter runs entirely on your own machine — your files are never uploaded to a
server. Drop a ICO file in and pick an output format, or jump straight to a common one:
16×16 and 32×32 cover the address bar, tabs, and bookmarks on virtually every browser. Adding 48×48 helps with Windows shortcuts. Anything larger is better supplied as a separate PNG referenced in your markup rather than bloating the ICO.
Can I just rename a PNG to .ico?
No. The ICO container has its own header and directory structure, and renaming the file does not create one. Browsers will often still display it, because they sniff the actual contents, but Windows and many other tools will not. Convert it properly.
Do I still need an ICO file at all?
Strictly, no — browsers accept PNG and SVG favicons declared in your markup. But browsers and crawlers also request /favicon.ico automatically without reading your markup at all, so having one avoids a stream of 404s and covers the tools that only look there.
How do I convert a logo to ICO?
Start from a square source — SVG or a large PNG — and convert to ICO with the standard sizes included. If your logo is wide rather than square, crop or simplify it to a square mark first; squeezing a wordmark into 16×16 pixels produces something illegible.
