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.
How to open 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. For a step-by-step walkthrough, see how to open a .ico file.
Windows
- IrfanView
- GIMP
- Paint.NET
- browsers
macOS
- Preview (view only)
- GIMP
Linux
- GIMP
- Image Viewer
Web
- Browsers, as favicons
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
.ico file signature (magic bytes)
Programs recognise a .ico file by the bytes at the start of the file, not by its name. These are the signatures for ICO. For a full breakdown, see the ICO file signature page.
| Hex signature | Offset | 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.