Favicon Generator
Resize one image to the four sizes that actually get used, and get the link tags.
HTML to paste
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180 px preview (Apple touch icon)
Worked example
Four sizes cover everything that matters: 16 for browser tabs, 32 for bookmarks and taskbar, 180 for iOS home screen, 192 for Android. Anything beyond that is unused weight.
A favicon is 16 pixels wide. Detail disappears at that size, so the source image should be a simple mark with strong contrast rather than a full logo with text.
Square or it gets cropped
The canvas crops to the centre square before scaling. A wide logo will lose its edges — that is better than squashing it, but the real fix is to upload a square source.