What is a .ico file?
Windows Icon file · Image · Microsoft · 1985
ICO is the Windows icon format and the classic choice for a website favicon. A single ICO can bundle the same icon at several sizes, so the system or browser picks whichever resolution it needs.
What is a .ico file used for?
An ICO file packs one or more small images at sizes like 16, 32 and 48 pixels. Windows uses it for program and folder icons, and browsers use favicon.ico for the little tab image.
Because it can hold multiple sizes in one file, an ICO stays sharp whether it is shown tiny in a tab or larger on the desktop.
For anything beyond icons it is the wrong tool. Use PNG or SVG for general graphics.
What opens a .ico file?
The .ico format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.
Windows
- IrfanView
- GIMP
- Paint.NET
- browsers
macOS
- Preview (view only)
- GIMP
Linux
- GIMP
- Image Viewer
Web
- Browsers, as favicons
Open a .ico file step by step
- Confirm the file really is a .ico. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
- On Windows, open it with IrfanView or GIMP.
- On Mac, use Preview (view only) or GIMP.
- On Linux, try GIMP or Image Viewer.
- Still stuck? Convert the .ico to PNG, which opens more widely.
Why can't I open my .ico file?
Almost always because no installed app understands ICO. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.
The other possibility is that the file is not really a .ico at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.
How to convert a .ico file
You can convert a .ico file to PNG using a conversion tool or the export menu of an app that opens it.
Convert images free: ImageConvert
What is the .ico file signature (magic bytes)?
A .ico file starts with the hex bytes 00 00 01 00. Programs identify ICO by those bytes, not by the filename, so a renamed file still gives itself away.
These are the documented signatures for ICO. For a full breakdown, see the ICO file signature page.
| Hex signature | Offset | ASCII | Note |
|---|---|---|---|
| 00 00 01 00 | 0 | - | icon resource |
See the full magic-numbers reference for every format.
Frequently asked questions
What is a .ico file?
ICO is the Windows icon format and the classic choice for a website favicon. A single ICO can bundle the same icon at several sizes, so the system or browser picks whichever resolution it needs.
How do I make a favicon ICO?
Create a square PNG at 32x32 or larger, then convert it to ICO. Many ICO files include several sizes in one so the browser can pick the best.