The favicon format browsers and Windows actually expect
The .ico format has outlived every prediction of its death because two major consumers still expect it: browsers, which by long convention request /favicon.ico from every site, and Windows, which uses ICO for application, shortcut and folder icons. Modern HTML lets you declare PNG or SVG favicons, but the plain favicon.ico in the site root remains the lowest common denominator that old browsers, RSS readers, bookmark services and search engine crawlers all understand. This tool builds that file properly: each size inside it is PNG-compressed, which keeps the container small while preserving the alpha channel your logo needs. If your logo currently only exists as a bitmap, consider tracing a vector master first with PNG to SVG, and use SVG to PNG to export a crisp 256 pixel square as the input here.
Small sizes are where icons live or die
A favicon spends most of its life at 16 by 16 pixels, which is a brutally small canvas: roughly the size of a single letter of body text. Automatic downscaling handles the mechanics, and pre-rendering each size into the ICO means the browser never has to improvise, but design still matters. Fine line detail, small text and thin strokes vanish at 16 pixels; bold, simple shapes with strong contrast survive. If your full logo is a wordmark, the usual practice is to use just the mark or the first letter for the icon. Test the downloaded file by setting it as a favicon and looking at an actual browser tab, not a zoomed preview. To inspect what ended up inside any ICO, or to recover artwork from an existing favicon, ICO to PNG extracts the largest image, and JPG to ICO covers photographic sources.
Common questions
Why does one .ico file contain several sizes?+−
Because icons are displayed at many sizes and scaling one image to all of them looks bad at the small end. An ICO is a container: this tool packs 16, 32, 48, 64, 128 and 256 pixel versions into a single file, and the browser or Windows picks whichever fits the context, a 16 pixel tab icon, a 32 pixel taskbar icon, a 256 pixel desktop tile.
What if my PNG is smaller than 256 pixels?+−
The ICO simply includes the sizes at or below your source dimensions. Images are never upscaled, because inventing pixels would only produce a blurry icon that looks worse than omitting the size entirely. A 64 pixel source yields an ICO with 16, 32, 48 and 64 pixel entries. For the sharpest result at every size, start from a 256 pixel or larger square PNG.
Does my PNG need to be square?+−
No. A non-square image is centered on a transparent square canvas rather than stretched, so your logo keeps its proportions and the padding stays invisible wherever the icon is shown. That said, icons are displayed in square slots, so artwork designed square, with the subject filling most of the frame, will read better at 16 pixels than a wide wordmark ever can.
Can I just rename a PNG to favicon.ico instead?+−
It often appears to work in modern browsers, which sniff the real format, but it is a gamble. Windows will not accept a renamed PNG as an application icon, older browsers expect genuine ICO data, and you lose the multi-resolution behavior entirely, so the browser downscales one image on the fly. A real ICO with pre-rendered sizes is what the format exists for.