Overview
ICO dates back to Windows 1.0 (1985). The file contains a small directory followed by images at different sizes — originally uncompressed bitmaps with a transparency mask and, since Windows Vista, PNG-compressed images of up to 256 × 256 pixels.
Browsers adopted favicon.ico in 1999. Modern browsers also accept PNG and SVG favicons, but many tools and crawlers still request /favicon.ico by default, so shipping one remains good practice.
Advantages
- +Multiple resolutions in one file
- +Favicon support in every browser
- +Alpha transparency
Disadvantages
- −Maximum 256 × 256 pixels
- −Not meant for general images
- −Bitmap-based entries are large
How ICO compression works
Each entry is either an uncompressed device-independent bitmap with a 1-bit mask or a complete PNG file. JFIF.IO writes PNG-compressed entries, supported by Windows Vista and later and by all browsers.
Common use cases
- Website favicons
- Windows application and shortcut icons
Browser and software support
All browsers read ICO favicons; most also display .ico files opened directly.
How to convert ICO
Use the converter at the top of this page: drop your file, choose an output format and download the result. Conversions run in your browser, so nothing is uploaded.
- JPG has no transparency. Transparent areas will be filled with the background colour.
Processed locally in your browser
ICO FAQ
Which sizes should a favicon.ico contain?
16×16, 32×32 and 48×48 cover browser tabs, bookmarks and Windows shortcuts. Add a 180×180 PNG for Apple devices and 192/512 PNGs for Android — the favicon generator creates all of them.
Do I still need favicon.ico?
It is not strictly required if you declare PNG or SVG icons, but many tools and crawlers request /favicon.ico automatically, so it is worth including.