ICO vs PNG: Choosing the Right Format for Icons and Favicons
ICO is a format built specifically for one job: application icons and website favicons, particularly on Windows. PNG is the general-purpose image workhorse that also happens to work fine for that same job today. Understanding what ICO actually adds over a plain PNG clarifies when the extra format-specific step is genuinely worth it.
What makes ICO specifically different
An ICO file's defining feature is that it can bundle multiple image sizes and color depths into a single file — one ICO can contain a 16x16 version, a 32x32 version, a 48x48 version, and more, all packaged together. The operating system or browser then automatically picks whichever size fits the specific context it needs (a taskbar, a desktop shortcut, a browser tab), all from that one bundled file, rather than needing separate files for each size.
Why this mattered more in the past
This all-sizes-in-one-file approach was genuinely valuable for Windows application icons, where the same icon needs to display correctly at several different sizes across different parts of the operating system interface, and having one file handle that automatically was a real convenience over managing several separate image files.
Why modern browsers changed the calculation for favicons
Modern web browsers have become considerably more flexible about favicon formats — a plain PNG (or even an SVG, in supporting browsers) works perfectly well as a favicon today, without needing the multi-size bundling ICO provides. This is a meaningful shift from years ago, when ICO was essentially mandatory for a favicon to display correctly across different browsers; today, a single well-sized PNG favicon works reliably in virtually all modern browsers without any special format requirement.
Where ICO still genuinely matters
For actual Windows application icons — the icon shown in a taskbar, a desktop shortcut, an application's file explorer entry — ICO remains the expected, often required format, since Windows' icon-display system was built specifically around ICO's multi-size bundling capability. This is a genuinely different context from a website favicon, where browser flexibility has made the requirement much looser than it used to be.
A practical way to decide
For a website favicon today, a properly sized PNG works fine in virtually all modern browsers — the ICO requirement that used to be closer to mandatory has substantially relaxed. For an actual Windows application icon, ICO remains the standard and often required format, since that's specifically what Windows' icon system expects. If you're building a favicon and want maximum backward compatibility with genuinely old browsers, ICO still doesn't hurt, but for a modern site it's no longer strictly necessary the way it once was.
The image converter and Resize Image handle preparing an image at the right size and format for either a modern PNG favicon or a properly sized icon file, depending on which context you're actually building for.