What ICO is
ICO is Microsoft's icon format — a container that bundles several image sizes into a single file. The most famous example is the favicon.ico in a website's root directory: browsers automatically look there for a tab icon. An ICO can hold, say, 16-, 32-, and 48-pixel versions of the same motif, so the system picks the right one for the display.
Do you even need an ICO?
Honest answer: not strictly, for modern browsers — a PNG favicon is enough. But a favicon.ico in the root directory stays a sensible fallback:
- Some services and feed readers look specifically for
/favicon.ico. - Older software expects the classic format.
- It "just sits there" and is found even without an HTML tag.
It never hurts to provide it alongside the PNG variants — the full favicon set is described in Creating a favicon.
Which sizes belong in it
A favicon.ico needs only a few sizes:
- 16 × 16 — the browser tab.
- 32 × 32 — sharper display, Windows taskbar.
- 48 × 48 — optional, for larger displays.
Larger resolutions do not belong in the ICO — they'd bloat the file that's requested on every page load. Deliver large icons as separate PNGs and Apple touch icons.
Making an ICO from a PNG
The easiest route: a favicon generator. You supply a square, simple motif (a filigree logo turns to mush at 16 pixels — reduction is mandatory), and the generator creates the right sizes, bundles them into the ICO file, and delivers the accompanying PNGs plus the HTML code. It runs browser-local, without your logo being uploaded.
The design detail
The most important advice has nothing to do with the format: at 16 pixels only a heavily reduced motif survives — a letter, a basic shape, two colors. Force your detailed logo into the ICO unchanged and you get an unrecognizable blob in the tab. Simplify first, then generate.
In short
- ICO bundles several icon sizes in one file.
- Not strictly needed today, but a good root-directory fallback.
- 16/32/48 px only — keep the file small.
- Simplify the motif before generating from a PNG.