Not every .ico file is the same kind of file. Some bundle several sizes of the same icon together in one package, ready for Windows to pick whichever fits a given context. Others hold exactly one image at one fixed size, wrapped in an ICO container because that's the file type something is asking for. This converter builds the second kind: it takes your PNG and packages it as a single-frame ICO at the PNG's own dimensions, rather than generating a full multi-size set inside one file.

That's not a shortcut or a limitation to apologize for. A single-size ICO is a completely valid file and it's genuinely all a lot of situations call for, particularly a website favicon, which almost always just needs one reasonably sized icon rather than a full Windows icon-set architecture. But it does mean the source PNG you start with matters more than it would with a multi-size tool doing the resizing for you.

Visual Comparison: 128×128 Direct Conversion

Here's what that looks like in practice. We converted a 128×128 PNG straight to ICO with no resizing involved.

Same image, same dimensions, and the ICO version comes out with visibly rougher, stair-stepped edges around the curves, something the PNG didn't have. This isn't a resizing problem since nothing got scaled up or down here. It comes down to how the ICO format traditionally handles transparency: unlike PNG's smooth, gradual alpha channel, the classic icon format many systems still expect stores transparency as a harder edge, without the same anti-aliasing. Curved or diagonal lines that looked smooth in the PNG can pick up a slightly jagged look once they're packaged into that structure, particularly noticeable on artwork with soft gradients or rounded shapes like the one above.