Start from a square image
An ICO file is the Windows icon format, and each image inside it is at most 256 by 256 pixels. The format stores width and height in a single byte, where 0 stands for 256, so nothing bigger fits. Windows has drawn 256-pixel icons since Vista in 2007; the sizes you actually see on screen are 16, 32, 48 and 256 pixels.
The converter builds those sizes itself, from 16 up to 256 pixels, so there's no need to shrink the image first. What it doesn't do is change the shape. A 600 by 300 image gives a 256 by 128 icon, then 128 by 64, and so on down to 16 by 8. So cut the AVIF to a square with the crop tool before you convert. A logo or a simple symbol reads better at 16 pixels than a photo ever will.
How to convert AVIF to ICO
- Upload the cropped AVIF file. Up to 5 files, 300 MB each.
- Pick ICO as the target.
- Download the .ico file once it is ready, and name it favicon.ico if it is for a website.
Where the ICO goes
For a website, place favicon.ico at the root of the domain; browsers still request that path even when no link tag points to it. For a Windows shortcut, right-click the shortcut, open Properties and choose Change Icon. To go the other way, ICO to AVIF is also supported, and the ICO hub lists the rest.