.ico File Extension

What kind of file is a .ico?

An .ico file is a Microsoft Windows icon container. It can hold several raster versions of the same symbol at different dimensions and color depths, allowing software to choose a suitable representation for a menu, desktop, taskbar, or high-resolution display. Its registered MIME type is image/vnd.microsoft.icon; the older image/x-icon label remains common.¹

The identifying header begins 00 00 01 00. A related cursor file starts 00 00 02 00, so content inspection can distinguish an ICO from a CUR file even if the suffix is misleading.¹ Traditional entries contain device-independent bitmap data and a transparency mask. Newer ICO files can also embed complete PNG images.²

How do I open a .ico on my computer?

Windows uses ICO files directly for application, shortcut, and file-type icons. Chrome, Edge, Firefox, Opera, and Safari can display them,³ while GIMP can open and export the individual icon representations.⁴

• On Windows, the shell or a browser is enough to view an icon. Use GIMP or a dedicated icon editor to change it.
• On macOS and Linux, use GIMP or a current web browser.
• For development, Visual Studio can assign an ICO file as a Windows application's icon.⁵

A normal image viewer may show only one embedded representation. Use an icon-aware editor if you need to inspect or preserve every size and color depth.

Poly support for .ico

Poly creates thumbnails for ICO files, opens them in the image viewer, indexes their filenames and supported image properties, and enables visual-similarity search. The Poly agent can reason about the rendered artwork.

Poly's preview represents the icon as an image rather than exposing its internal directory of sizes. Text drawn into an icon is not indexed as document text. If exact differences between the 16 by 16 and 256 by 256 versions matter, inspect the original with an icon editor.

History of .ico

The suffix comes from Windows icon resources, which can contain more than one image.⁶ The format's little-endian fields reflect its origin on Intel-based systems. IANA registered image/vnd.microsoft.icon in 2003, standardizing a media type for files that had also circulated under names such as image/x-icon.¹

PNG-compressed entries arrived with Windows Vista. Microsoft recommended a full application-icon set of 16 by 16, 32 by 32, 48 by 48, and 256 by 256 pixels, with the 256-pixel image compressed to control file size.²⁷

Working with .ico and alternatives

Choose the format for the destination:

When converting ICO to PNG, choose the appropriate embedded representation or export each size separately. When creating ICO from PNG or SVG, rasterize and review every target size before assembling the container. A detailed design that looks good at 256 by 256 may become unclear at 16 by 16.

Treat downloaded icons as untrusted binary files. The IANA registration warns that malformed offsets can expose weaknesses in decoders, so use maintained software and avoid opening suspicious files with obsolete icon tools.¹