Quick Answer: An ICO file is a Windows icon file that holds one image in several sizes, including 16, 32, 48, and 256 pixels. Websites use it as a favicon, and Windows uses it for app icons. You can make one from a PNG or JPG.
What an ICO file is
An ICO file is a Windows icon file. You will see it with the .ico ending. It is not a photo format, and it is not a document format.
Wikipedia notes in the ICO file format on Wikipedia that an ICO file can hold several images in one file. That single fact explains most of the format. One file can carry a small image and a large image together.
An ICO file is a container, in plain words. Think of it as a small box with several pictures inside. A program opens the box and picks the picture that fits its need.
You may meet an ICO file in three common ways. You may download one, you may need one for a website, or you may want one for a Windows folder. Each use draws on the same idea.
Pro tip: Keep the source image you made the ICO file from. You will need it again if you want a different size or a fix.
Why one ICO file holds several sizes
A screen shows icons at many sizes. A tiny tab icon and a large desktop icon need different detail. One size stretched to fit every case would look blurry or crude.
An ICO file solves this by holding several sizes at once. A program picks the closest size for its spot. That choice keeps small icons sharp and large icons clear.
| Size | Typically used for |
|---|---|
| 16x16 | Browser tab favicon and small list views |
| 32x32 | Taskbar icons and shortcut icons |
| 48x48 | Desktop icons and larger list views |
| 256x256 | High-DPI displays and large icon views |
A 16-pixel icon must stay readable at a glance. A 256-pixel icon must stay clean when shown large. Holding both in one file serves both needs.
Pro tip: Check your icon at its smallest size first. If it fails at 16 pixels, simplify the design before you worry about large sizes.
ICO files as favicons
A favicon is the small icon in a browser tab. It helps a reader find your site among many open tabs. It also appears in bookmarks and browser history in many browsers.
An ICO file works well as a favicon because it can hold the small size a tab needs. It can also hold a larger size for other spots. One file can serve several places at once.
Many modern browsers also accept a PNG file as a favicon. An ICO file is still a safe choice when you want wide support. It is the older, widely known favicon format.
If your source is a PNG file, a PNG to ICO converter can turn it into an ICO file. Start from a square image with a clear, simple shape. Simple shapes read best at small sizes.
Pro tip: Test the favicon in a real browser tab. A design that looks good large can turn into a blur at tab size.
ICO files in Windows
Windows uses ICO files for app icons, folder icons, and shortcut icons. The same file can appear on the desktop, in the taskbar, and in a file list. Each spot picks a size that fits.
A folder icon is a good example. You may see it small in a list and large on the desktop. One ICO file with several sizes serves both views without a new file for each view.
Windows typically reads ICO files directly for these uses. A PNG file alone typically does not serve as a Windows app or folder icon in the same way. That gap is why people convert an image to ICO format.
If your source is a JPG file, a JPEG to ICO converter can turn it into an ICO file. Start from a square image, and check the result at each size you care about.
Pro tip: Apply the ICO file to one folder or shortcut first. Check it at small and large sizes before you change many icons at once.
Transparency and image quality
Transparency means parts of an image can be see-through. An ICO file typically supports transparency. That support matters for icons, which often float over varied backgrounds.
A good icon uses transparency to avoid a white box around its shape. A circle icon, for example, should show the desktop through its corners. A solid box behind it would look wrong.
Image quality in an ICO file depends on the source image. A sharp, square source gives a better result than a small, blurry source. Converting does not add detail that was not in the source.
Pro tip: View the icon over a dark background and a light background. A transparency problem hides on one and shows on the other.
How to open an ICO file
You can typically open an ICO file in Windows by double-clicking it. Windows typically shows a preview of the icon. That preview tells you the icon works, even if it does not open an editor.
An image viewer or editor may also open an ICO file, and support varies by program. If you need to view or edit the image as a normal picture, convert it first. An ICO to PNG converter can turn an ICO file into a PNG file. A PNG file typically opens in more editors and browsers.
Do not rename an ICO file to end in .png and expect it to work. A new ending does not change the data inside. Conversion changes the data to match the new format.
Pro tip: If a program refuses the file, try a preview first. A preview confirms the file itself, not just the program you tried.
How to make an ICO file, and when not to use one
Start from a square image with a simple shape. A PNG file is a common starting point, and a JPG file can also work. Check transparency before you convert, since a white box in the source can carry into the result.
A PNG to ICO converter and a JPEG to ICO converter are the direct routes from those formats. A vector source needs a different route, such as an SVG to ICO converter. For a fuller walkthrough of icon work, read the guide on how to convert icon files.
An ICO file is not the right choice for every image. Use a PNG file for a normal web image, a photo, or a picture you plan to edit. Use an ICO file when you need a favicon or a Windows icon.
Keep the source image after you make the ICO file. You may need it for a new size, a fix, or a different icon format later. The ICO file is the output, not the master.
Pro tip: Make the icon, then test it where it will live. A tab, a taskbar, and a desktop view can each reveal a different flaw.
How to make an ICO file
- Choose a square PNG or JPG file.
- Upload the file to a converter.
- Convert the file to ICO format.
- Download the ICO file and test it at small and large sizes.
Recap
- An ICO file is a Windows icon file that can hold several sizes in one file.
- Typical sizes are 16, 32, 48, and 256 pixels, for favicons, taskbar, desktop, and high-DPI views.
- ICO files typically support transparency, which keeps icons clean over varied backgrounds.
- You can make one from a PNG or JPG file, and convert one back to PNG when you need wider editing.
Frequently Asked Questions
What is an .ico file?
An .ico file is a Windows icon file. It typically holds one image in several sizes, such as 16, 32, 48, and 256 pixels. A program picks the size that fits where the icon appears.
How do I open an .ico file?
You can typically open an .ico file in Windows by double-clicking it. Windows typically shows a preview of the icon. If you need to edit it, convert it to a PNG file first, since PNG files typically open in more editors.
Why does one ICO file hold several sizes?
Icons appear at many sizes, from a browser tab to a large desktop view. One size stretched to fit every spot would look blurry or crude. Holding several sizes lets each spot use a sharp image that fits.
Does an ICO file support transparency?
ICO files typically support transparency. That means parts of an icon can be see-through, so a round icon does not need a white box behind it. Check the icon over dark and light backgrounds before you use it.
What is the difference between an ICO file and a PNG file?
A PNG file typically holds one image at one size. An ICO file typically holds several sizes of one image in a single file. PNG files are better for normal web images and editing, while ICO files are better for favicons and Windows icons.
How do I make an ICO file from a PNG or JPG file?
Start from a square image with a simple shape. Use a PNG to ICO converter for a PNG file, or a JPEG to ICO converter for a JPG file. Check the result at small and large sizes before you use it.
Can I use an ICO file as a website favicon?
Yes, an ICO file is a common favicon format. It can hold the small size a browser tab needs, and larger sizes for other spots. Test it in a real browser tab, since a design that looks good large can blur at tab size.
Can I convert an ICO file back to a PNG file?
Yes, you can convert an ICO file to a PNG file. That step helps when you need to view or edit the image in a program that does not open ICO files. Keep the original ICO file if you still need it as an icon.