PNG to ICO, and a proper multi-size favicon

A .ico file is a container that can hold several images at different resolutions. Windows and browsers then pick whichever one fits — 16×16 in a browser tab, 32×32 in the taskbar, 256×256 in Explorer’s large-icon view. A single-size icon gets scaled by the operating system and looks blurry everywhere it does not fit exactly.

This tool resizes your source image to each size you tick, then packs the results into one combined multi-size .ico, plus individual .ico and .png files for each size.

Notes that save time

Start from a square image — non-square sources get squashed into the icon box.
256×256 is the largest size the ICO format can address, so bigger sizes are exported as PNG only and left out of the combined icon.
For a website favicon, put favicon.ico at the site root; it is still the most widely supported option.
You can also feed an existing .ico back in — the largest layer inside it is used as the source.

Conversion happens locally in your browser; the images never leave your device.