Images

What is ICO?

Also called favicon.

ICO is the Windows icon format. One .ico file can hold the same icon at several sizes, which is why websites use it for their favicon.

By Utility Mule editorial team Last reviewed September 29, 2026 How we write guides

How it works

An ICO file is a small container of images. Each entry is the same icon at a different pixel size, stored as a bitmap or as a PNG. Programs choose the size that best fits the space available instead of scaling a single image up or down.

Websites inherited the format when browsers began looking for a file called favicon.ico at the root of each site. The favicon appears in tabs, bookmarks, history and search results, so it is often a visitor’s first glimpse of a brand.

Designing for the smallest size matters most. At 16 pixels there is only room for a letter or a simple shape, so many sites use a simplified mark for the favicon and the full logo elsewhere. A favicon generator produces the ICO along with the PNG sizes phones and tablets expect.

A practical example

A favicon.ico containing 16 × 16, 32 × 32 and 48 × 48 pixel versions lets the browser pick a crisp icon for a tab, a bookmark or a desktop shortcut.

What to watch out for

Scaling one large image down to 16 × 16 often turns fine text and thin lines into a blur. Simplify the design for the smallest size rather than shrinking a detailed logo.

Tools you can use

Favicon Generator · The whole icon set, plus the HTML to paste
Image Converter · Change an image from one format to another

Go a little deeper

Common questions

Do I still need a favicon.ico?

Modern browsers accept PNG and SVG favicons, but a favicon.ico at the site root is still the most compatible fallback and is requested automatically by many tools.

What sizes should a favicon include?

16 × 16 and 32 × 32 cover browser tabs. Add 48 × 48 for Windows shortcuts, a 180 × 180 PNG for Apple touch icons and 192 and 512 pixel PNGs for Android.

Can an ICO file be transparent?

Yes. ICO images can store an alpha channel, so a round logo can sit on any tab color without a white square behind it.