A container, not a single image
What makes ICO unusual is that one file holds many images at once. A directory at the start lists each entry with its width, height and bit depth, and points to where that image’s data lives further down. Each entry is stored internally as either a BMP-style bitmap or, in modern files, a complete PNG — the format happily mixes both. This design lets Windows render a crisp 16×16 icon in a list and a detailed 256×256 icon on the desktop from the same file, rather than scaling one source up and down badly.
Because the smaller bitmaps need clean edges, ICO supports an alpha channel for transparency, and older entries additionally carried a one-bit mask. Favicons reuse the same format, which is why the humble favicon.ico still works in every browser despite newer alternatives.
Sizes that actually matter
A well-built icon includes the sizes the platform asks for — commonly 16, 32, 48 and 256 pixels square — because each is used in a different place, from title bars to file dialogs. The 256-pixel entry is almost always a PNG to keep the file from ballooning. Designers craft the tiny sizes by hand rather than downscaling, since automatic shrinking turns sharp glyphs into mush at 16 pixels.
Strengths
- Holds many sizes in one file so the system always has a sharp match.
- Supports an alpha channel for clean, anti-aliased edges.
- Works as a favicon in every browser without special handling.
Limitations
- Aimed squarely at icons, not a general-purpose image format.
- Hand-tuning the small sizes is fiddly and easy to get wrong.
- The legacy BMP-based entries waste space compared with embedded PNGs.
When to use ICO
Use ICO when you need a Windows application icon or a classic favicon, packing the handful of standard square sizes your platform expects into one file.
ICO FAQ
- What sizes should a favicon.ico contain?
- A practical favicon holds 16×16, 32×32 and 48×48 pixel images, and often a 256×256 entry for high-resolution displays. Browsers pick whichever size suits the context, so including several avoids blurry scaling.
- Can an ICO file contain PNG images?
- Yes. Since Windows Vista the format allows each entry to be a full PNG, which is the usual way to store the larger 256-pixel icon compactly. Smaller entries may still use the older BMP-style storage.
- How do I turn a PNG into an icon?
- Start from a square, transparent PNG and convert it to ICO so the alpha edges stay clean. Morphr can rasterise to and from these formats in the browser, keeping the source file on your machine.