Renaming a PNG to .ico and assuming it works everywhere, when browsers often cope but Windows shell contexts expect a real ICO container.
File formats
ICO
Also called .ico, icon file or favicon file. Here is what it means, when it changes what you export, and what people get wrong.
File formatsRuns on your deviceUpdated 10 August 2026
The short answer
ICO is the Windows icon format, a small container that holds several sizes and colour depths of the same image inside one file. Whatever is drawing the icon picks the entry that fits the space it has. It remains the most broadly supported format for a site's favicon.
Updated
The short version
ICO at a glance
| Quick fact | Detail |
|---|---|
| Maximum entry size | 256 x 256 pixels |
| Entry encodings | Windows bitmap, or PNG on Vista and later |
| Common favicon sizes | 16, 32 and 48 pixels square |
| Transparency | Yes, 8-bit alpha in 32-bit entries |
| Default request path | /favicon.ico at the site root |
| Animation | Not supported |
Slide the table sideways to see every column.
How does one file hold several sizes?
An ICO file opens with a short directory listing every image inside it, giving each one its width, height, colour depth and position. A viewer reads that list, picks the closest match to the size it needs and decodes only that entry. So a single favicon.ico can serve a 16 pixel browser tab and a 48 pixel desktop shortcut without either being a scaled-down guess.
Each entry is stored either as a Windows bitmap with a separate transparency mask, or, on Windows Vista and later, as PNG data. PNG entries are how larger icons stay a sensible file size. The width and height fields are single bytes, so 256 x 256 is the largest an entry can be.
Which sizes should a favicon.ico contain?
16, 32 and 48 pixels square covers the practical cases: 16 for a browser tab, 32 for higher-density tabs and taskbar use, 48 for Windows shortcuts and file listings. Larger icons for app installs and mobile home screens are normally shipped as separate PNG or SVG files referenced in the page head rather than packed into the ICO.
Simplify the artwork before you shrink it. Fine strokes, gradients and small type vanish at 16 pixels, so tab icons usually want a bolder, cropped version of a logo rather than the logo itself. Resizing the source cleanly to each square first (Photomop's resizer is one way) then packing those squares beats letting one large entry be downscaled on the fly.
Is ICO still worth shipping?
Yes, as a fallback. Browsers request /favicon.ico from the root of a domain even when no markup points at it, so an ICO there is the one icon found without any HTML at all. It also covers Windows contexts, such as pinned shortcuts, that expect a genuine icon container. Modern sites pair it with explicit PNG or SVG icons declared in the page head, which handle high-density displays and dark mode variants more gracefully.
What people get wrong about ico
Each one is a real failure mode, not a style preference.
Packing only a 256 pixel entry and letting every context downscale it, which looks soft and muddy at 16 pixels.
Shrinking a detailed logo with thin strokes to 16 pixels without simplifying it first, so the tab icon becomes a smudge.
Serving the icon with a long cache lifetime and then wondering why an updated favicon does not appear for weeks.
Questions people ask
ICO, answered
The follow-up questions people search for once they have the definition.
Do I still need a favicon.ico file?
It is still worth having. Browsers look for /favicon.ico at your domain root automatically, so it works even when the markup in your page head is missing or wrong. Most sites now ship it alongside PNG and SVG icons declared explicitly for higher-density screens.
What size should a favicon be?
Include 16, 32 and 48 pixel square entries. 16 is the classic tab size, 32 covers higher-density displays and Windows taskbar use, and 48 appears in shortcuts and file listings. Larger app icons are better supplied as separate PNG files.
Can an ICO file have a transparent background?
Yes. 32-bit entries carry a full 8-bit alpha channel, so edges can be smoothly anti-aliased against any background. Older low colour depth entries use a one-bit mask instead, which gives hard, jagged edges rather than soft ones.
Can an ICO file be animated?
No. ICO holds multiple sizes of a still image, not frames of an animation. Windows uses a separate format for animated cursors, and animated favicons on the web are done with GIF, APNG or SVG instead.
Same cluster
More from the glossary
Neighbouring entries, so every term in the set is one hop from every other.
Image matting
Image matting works out how transparent every pixel along an edge is, and what colour the subject would be there without the background showing through.
ICC profile
An ICC profile is a standardised file that describes the colour behaviour of a device or a colour space.
Image metadata
Image metadata is the information stored inside an image file about the image rather than in the pixels: camera settings, dates, location, captions, credit, keywords and colour profile.
From the studio
Tools that do this
Each one runs on your own device, so the photo stays where it is.
Sources
Where the facts on this page come from. Every link opens the specification, standard or documentation the claim was read at.
Keep reading
Elsewhere on Photomop
Platform image rules
Image size, file type and background rules for 22 selling platforms, each one sourced.
Read itConvert image
JPG, PNG, WebP and AVIF, with an honest answer about HEIC and alpha.
Read itAll the tools
The whole toolbox in three families, with what each one needs before it can run.
Read itOn-device privacy
The pipeline in five steps, what leaves your device, and how to check it yourself.
Read it
Now do it to a photo
Photomop is a photo studio that runs on your own device. Resize, crop, compress, convert, batch edit, remove a background and change one all work in the browser tab you are reading this in, at full resolution and with no watermark. The on-device page shows you how to check that the photo stays put.