Convert PNG to ICO

Almost everyone converting a PNG to an ICO is making a favicon. Browsers have requested `/favicon.ico` from the root of every site for nearly thirty years and they still do, regardless of what your HTML declares — so having one there stops a stream of 404s in your logs and covers the tools that look for nothing else.

Modern favicon markup can point at a PNG or an SVG as well, and most sites ship both. The ICO is the fallback that never has to be declared.

A container rather than a compression format, which is why this conversion loses nothing meaningful. Your image is stored inside it as a PNG — one per size in the directory — complete with its alpha channel, so a rounded logo or a mark with transparent corners keeps them exactly.

One constraint comes from the format and it is a hard one: an icon caps at 256 pixels, so anything larger is scaled down. The size byte in an ICO directory entry is a single byte, and 256 is written as zero because it does not fit.

Underneath that ceiling you choose. The "Icon sizes" control decides what goes into the directory, and the default writes three entries — 48, 32 and 16 pixels, largest first — each stored as its own PNG. That is the arrangement a favicon has wanted since the beginning: the browser reads the directory and takes the entry closest to the size it is about to draw, instead of reducing one large image on the fly with a general-purpose filter.

Every entry written here is square, because icons are square. A non-square source is not cropped and not stretched: it is centred on the square at the largest scale that fits, with transparent space filling the rest. Nothing is distorted, but an oblong logo ends up drawn across half the icon and looks smaller than its neighbours in a row of tabs.

So square it up in whatever tool made it, at 256 pixels or more, and let the mark fill about ninety per cent of the frame. That is one edit in the source and it decides more about how the icon reads than anything on this page.

16 and 32 pixels are the ones that matter in practice: 16 for a browser tab and a bookmark list, 32 for the taskbar and most desktop contexts. Windows also draws icons at 48 and at 256 in certain views.

Those are the three the default set writes, which is why it is the default. Start from a large source anyway: every entry is reduced from the picture you drop in, so a 256-pixel PNG produces a clean 48, 32 and 16, while a 32-pixel source asked for 256 is enlarged and looks exactly as bad as that sounds.

The constraint that catches people is not the format, it is the arithmetic. At 16 pixels a letterform is about ten pixels tall, which is not enough for a wordmark, a tagline, or any stroke thinner than the rest.

What works is one shape, high contrast, few colours: the initial rather than the name, the symbol rather than the full lock-up. Test it by shrinking the PNG to 16 pixels before converting — if you cannot tell what it is, no format will rescue it.

Put it at the root of the site as favicon.ico. That single placement covers browsers requesting it directly, feed readers, and older tools that look nowhere else.

Declaring it in the HTML head is worth doing as well, alongside a PNG for current browsers and an apple-touch-icon for iOS home screens. The ICO is the floor, not the whole strategy.

Almost always caching, and favicons are cached unusually hard — by the browser, by any CDN in front of the site, and sometimes by the operating system as well.

Load the icon URL directly to confirm the new file is being served, then clear the site data for that one origin rather than the whole browser. If a CDN sits in front, purge it there too. Once the file itself is confirmed correct, the rest is patience.

Drop up to a hundred PNGs in one pass. Each becomes its own ICO and they come back together as a ZIP, which suits an app or a set of sites needing one icon each.

The conversion runs in your browser, so there is no upload, no queue and no daily allowance. The other cap is 100 MB a file, and icons are small enough that this is never the constraint.