The short answer
- ICO is a Windows icon container that stores multiple bitmap images with different sizes and color depths, with applications choosing the version best suited to the display environment.
- On Windows, open the ICO with an application that uses the native Windows Imaging Component ICO decoder. WIC provides native decoding but no native ICO encoder, so an application that can display the file may still be unable to create or save it. For Mac or mobile devices, choose an image application that explicitly lists ICO decoding; if it cannot read the container, convert a trusted copy while accounting for every embedded image.
- An ICO file requires parsing and is not safe merely because its extension or header looks correct. NVD records CVE-2004-1049, in which crafted image-dimension fields could trigger an integer and buffer overflow in Windows LoadImage. For untrusted files, limit file and image dimensions, validate directory offsets, and use an updated, isolated decoder.
ICO Image File: How to Open, Check, and Convert It
Microsoft formally calls ICO the Icon Format. An ICO file can hold several bitmap representations of one icon, including versions such as 16×16, 32×32, 48×48, and 256×256. Each image uses a bitmap and a mask for transparent areas; 32-bit icons can also use alpha transparency. Its identifying bytes are 00 00 01 00 at offset 0.
When converting a multi-image ICO to PNG or another ordinary image format, exporting only one image discards the remaining size and color-depth variants. When converting PNG to ICO, resizing or reducing the image to an 8-bit or 4-bit palette can introduce jagged edges and lose transparency or color detail. Preserve separate target sizes and alpha transparency where required.
How to open it
On Windows, open the ICO with an application that uses the native Windows Imaging Component ICO decoder. WIC provides native decoding but no native ICO encoder, so an application that can display the file may still be unable to create or save it. For Mac or mobile devices, choose an image application that explicitly lists ICO decoding; if it cannot read the container, convert a trusted copy while accounting for every embedded image.
On Windows, open the ICO with an application that uses the native Windows Imaging Component ICO decoder. WIC provides native decoding but no native ICO encoder, so an application that can display the file may still be unable to create or save it. For Mac or mobile devices, choose an image application that explicitly lists ICO decoding; if it cannot read the container, convert a trusted copy while accounting for every embedded image.
When converting a multi-image ICO to PNG or another ordinary image format, exporting only one image discards the remaining size and color-depth variants. When converting PNG to ICO, resizing or reducing the image to an 8-bit or 4-bit palette can introduce jagged edges and lose transparency or color detail. Preserve separate target sizes and alpha transparency where required.
File safety
An ICO file requires parsing and is not safe merely because its extension or header looks correct. NVD records CVE-2004-1049, in which crafted image-dimension fields could trigger an integer and buffer overflow in Windows LoadImage. For untrusted files, limit file and image dimensions, validate directory offsets, and use an updated, isolated decoder.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
When converting a multi-image ICO to PNG or another ordinary image format, exporting only one image discards the remaining size and color-depth variants. When converting PNG to ICO, resizing or reducing the image to an 8-bit or 4-bit palette can introduce jagged edges and lose transparency or color detail. Preserve separate target sizes and alpha transparency where required.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is an ICO file format used for?
An ICO file is a Windows icon container. It stores one or more bitmap images at different sizes and color depths so the system or an application can select an appropriate version for its current display environment.
How do I open an ICO file?
Use an image application with an ICO decoder. Windows Imaging Component includes native ICO decoding, although it does not include native ICO encoding; viewing support therefore does not guarantee that the application can create or save ICO files.
How do I open an ICO file on Windows 11?
Choose an application that uses the native Windows Imaging Component ICO decoder and open the .ico file with it. Windows can also extract icons from standalone ICO, EXE, and DLL files through its icon-extraction interfaces.
How do I open an ICO file on a Mac?
Choose a Mac image application that explicitly supports ICO decoding. Check whether it exposes every embedded size rather than showing only one representation; if it does not support ICO, convert a trusted copy with a suitable converter.
How do I view an ICO file on a phone?
Use a mobile image application that explicitly lists ICO decoding. If it cannot read the container, convert a trusted copy to PNG, selecting the needed embedded image and remembering that one PNG will not preserve the other ICO variants.
Why will my ICO file not open or appear correctly?
The application may lack an ICO decoder, the directory offsets or image data may be invalid, or it may be selecting an unsuitable embedded size. Verify the 00 00 01 00 header at offset 0, try an updated decoder, and do not repeatedly process an untrusted malformed file.
Should I use ICO or PNG for a Windows icon?
Use ICO when one Windows icon needs multiple sizes and color depths in a single container. PNG is an ordinary single-image choice; exporting only one ICO image to PNG loses the other embedded variants. The difference between ICO and PNG is that ICO is a multi-image container while PNG holds one image; keeping only one size forces the system to scale it, which can reduce sharpness.
How do I convert ICO to PNG without losing icon sizes?
Extract every required embedded ICO image and save each as a separate PNG. Exporting just one representation cannot retain the remaining sizes or color-depth variants in the multi-image ICO.
Is an ICO email attachment safe to open?
Do not trust it from its .ico name or header alone. Crafted ICO data has previously exploited a Windows image parser; treat an unexpected attachment as untrusted and use an updated, isolated decoder with size, dimension, and offset checks.
What sizes and transparency can an ICO file contain?
An ICO can contain multiple bitmap images with different sizes and color depths. Windows application icons commonly include 16×16, 32×32, 48×48, and 256×256 versions; masks create transparent areas, and 32-bit images may use alpha transparency.
Related formats
Sources
ICO Format Overview · Microsoft Learn（格式开发者的官方 Windows 文档）
Icons (Menus and Other Resources) · Microsoft Learn（官方 Win32 资源文档）
Icons (Design basics) · Microsoft Learn（官方 Windows 图标设计与兼容性指南）
IANA Media Types Registry · IANA（官方媒体类型注册表）
PRONOM Icon File Format Signatures · 英国国家档案馆 PRONOM（权威格式识别注册库）
NVD CVE-2004-1049 · 美国国家标准与技术研究院国家漏洞数据库