An ICO file is an image file format primarily used for computer icons in Microsoft Windows and for website favicons (the tiny icon you see in your browser tab). But what makes it different from a standard PNG or JPG?
The Magic of ICO: Multiple Sizes, One File
The main superpower of an ICO file is that it acts like a container. A single ICO file can contain multiple versions of the same image at different sizes (e.g., 16x16, 32x32, 64x64, 256x256). When you set an ICO as a desktop icon, the operating system automatically reads the file and displays the size that best fits your current view settings.
Why Do Websites Use ICO for Favicons?
While modern browsers support PNG favicons, favicon.ico remains the universal fallback standard. Older browsers only recognize the ICO format. By putting a multi-size ICO file in your website's root directory, you guarantee that every browser, old or new, will display your brand's tab icon correctly.
How to Create an ICO
You cannot simply rename a .png file to .ico. That will corrupt the file. You must use a dedicated converter tool that properly reconstructs the image headers into the ICO format.