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ICO
What is an ICO file?
Icon container holding several sizes in one file. Still what browsers expect as a favicon.
What ICO is
ICO is a container: a wrapper that holds streams encoded by something else. It stores a grid of pixels, so enlarging it past its own resolution softens it. It is used for the web.
The extension is .ico, and the full name is Windows Icon. Both matter less than what the file can hold, which is what the rest of this page is about.
Where ICO came from
Microsoft published it in 1985.
A format that has been readable for that long is a format worth trusting with something you want back in ten years.
The specification is published, but by its owner
It is written down and implementable, though the vendor decides what the next version looks like. In practice that is enough for the file to keep opening; it is not enough to be sure of it.
It is a specialist format
It exists to do one job well for one group of people. Outside that job it is a poor default, and inside it there is usually nothing better.
Nothing is thrown away
ICO stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.
It can be transparent
ICO carries an alpha channel, so a logo keeps its soft edges over whatever sits behind it instead of arriving in a white box.
Colour it can describe
Up to 8 bits per channel. It can work in RGB and an indexed palette.
Eight bits per channel is what a screen shows and what nearly every delivery format carries.
There is a size limit worth knowing
ICO cannot describe an image larger than 256 pixels on a side. That is far beyond an ordinary photograph and well within reach of a stitched panorama or a high-resolution scan, which is exactly when people meet it.
It can hold several pages in one file
A ICO file is not limited to one page, which matters when converting to something that is: a target that holds one image produces one file per page, and the numbering it chooses is worth checking before you send it on.
What opens ICO
GIMP and IcoFX read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
Every current browser reads it.
That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.
It is a delivery format
ICO is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
The facts, in one place
| Extension | .ico |
|---|---|
| Media type | image/x-icon, image/vnd.microsoft.icon |
| Published by | Microsoft |
| First published | 1985 |