ICO File Format (.ico)

An ICO file packs several copies of the same picture, each a different pixel size and color depth, into one Windows icon container so the operating system can pick whichever copy fits the taskbar, the desktop, or a File Explorer thumbnail.

.icoImage Files

ICO at a glance#

Full name	Microsoft icon
File extension	.ico
Category	Image Files
Developer	Microsoft
First released	1985
MIME types	image/vnd.microsoft.iconimage/x-icon
Compression	Varies from file to file
Specification	Microsoft ICO/CUR icon and cursor format
FileFlip support	FileFlip reads and writes this format

What a .ico file can hold#

These are the things a ICO file either supports or does not, and they are what decides whether a conversion keeps everything or quietly drops something.

Transparency (alpha channel)	Yes
Animation	No
Lossless compression	Yes
Embedded metadata	No
Colour profiles	No
High dynamic range	No
Multiple images or pages	Yes

Strengths and limitations of ICO#

What ICO does well

Holds every size and color depth a Windows icon needs in one file, from a 16x16 taskbar icon to a 256x256 Explorer thumbnail
Supports full 32-bit color with an 8-bit alpha channel, so edges anti-alias cleanly over any background
Windows Vista onward allows large frames to use PNG compression, keeping high-resolution icons small
The multi-image ICONDIR structure, dating to Windows 3.0, still opens in every version of Windows released since

Where ICO falls short

Capped at 256x256 pixels, so it cannot serve as a source file for the larger app icons modern platforms expect
Carries no metadata and no color profile, only raw image and mask data for each frame
Outside Windows and browser favicons, most software treats ICO as a niche format rather than a general image type
Editing one requires a dedicated icon editor rather than an ordinary photo editor, since most of those cannot write the multi-frame container

Use ICO for Windows application icons, desktop shortcuts, and website favicons, where the operating system or browser needs to choose the right size automatically.

Convert ICO to PNG when the image needs to be edited freely, resized to a size the icon doesn't already contain, or used anywhere other than Windows or a browser favicon.

How to open a .ico file#

A .ico file opens in the programs below, grouped by the platform you are on. If you would rather not install anything, FileFlip converts ICO into a format your machine already opens, and it does it in the browser tab rather than on a server.

Platform	Programs that open it
Windows	IcoFX, Greenfish Icon Editor Pro, GIMP, IrfanView
macOS	Image2icon, GIMP
Linux	GIMP, ImageMagick
Web	FileFlip

ICO questions people ask#

What is an ICO file?

An ICO file is the icon format Windows uses for application icons, desktop shortcuts, and browser favicons. Rather than holding a single picture, it bundles multiple versions of the same image at different pixel sizes and color depths so the system can display whichever one fits the context.

Why does one ICO file contain multiple images?

An ICO file contains multiple images because Windows needs a different size for a taskbar icon, a desktop icon, and a File Explorer thumbnail, and it would look blurry if a single small image were just stretched up. The multi-image ICONDIR structure lets one file carry the 16x16, 32x32, 48x48, and 256x256 versions together, and Windows picks the one closest to what it needs.

Can ICO files be animated?

No, the ICO format has no concept of animation frames or timing; it only stores static images at different sizes. Animated Windows cursors use a separate format, ANI, which is built on the RIFF container rather than ICO.

What's the difference between ICO and CUR files?

ICO and CUR share nearly the same internal layout, but a CUR file adds a hotspot coordinate that marks the exact pixel a mouse pointer clicks from, which an icon has no use for. That single difference is why Windows treats them as separate formats even though the underlying structure is almost identical.

What size should a favicon ICO be?

A favicon ICO typically bundles 16x16, 32x32, and sometimes 48x48 pixel versions in one file, covering browser tabs, bookmarks, and desktop shortcuts. Modern browsers also accept a plain PNG or SVG favicon, but ICO remains the one format every browser back to the earliest versions recognizes without any fallback markup.

Converting ICO files#

Every conversion runs inside your browser. The file is read from your disk, the work happens on your own machine, and nothing is uploaded. How FileFlip works names the engine behind each family and how to verify that for yourself.