GIF is a palette of at most 256 colours, all-or-nothing transparency, and animation. ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. ImageMagick sits between the two, and this page says exactly what happens to a GIF file on the way to becoming an ICO one.
What runs when a GIF file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your GIF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. GIF to ICO is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What GIF to ICO costs you
Every frame of the GIF but the first. GIF can hold an animation and ICO cannot, so frame one is what comes back. The alternative would be one numbered ICO file per frame and no single answer to hand you.
Any GIF bigger than 256 pixels a side. That is not a setting, it is the ICO file structure: width and height each live in a single byte. The engine scales larger sources down, and only down, so a phone photo becomes a 256 pixel icon.
If a model is going to read the ICO
An ICO file is pixels, and pixels are expensive for a model to read. Handed this ICO, an assistant spends its budget looking at the picture rather than reading it, and it will paraphrase what it thinks it saw. If your GIF is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
GIF to ICO, measured rather than promised
Running GIF to ICO against the real engine with a real file, ImageMagick wrote 202,814 bytes of ICO in under a tenth of a second, machine otherwise idle. That is one GIF file on one day and not an average, which is why the number is given together with the file that produced it.
The GIF to ICO verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the ICO file has anything inside it.
The same GIF file, sent somewhere else
Other outputs the probe measured out of a GIF file, so the cost of choosing ICO can be read against something. Sizes do not compare across engines, because each family was probed with its own GIF witness file.
GIF to PDF: 1,092,578 bytes in 0.9 seconds, verified by a format signature.
GIF to JPG: 20,559 bytes in under a tenth of a second, verified by a format signature.
GIF to PNG: 3,894 bytes in under a tenth of a second, verified by a format signature.
GIF to TIFF: 848,536 bytes in 0.2 seconds, verified by a format signature.
GIF to MP4: 54,682 bytes in 0.3 seconds, verified by ffprobe.
GIF to WEBM: 49,644 bytes in 2.0 seconds, verified by ffprobe.
Other ways into ICO, and what they measured
Among the published routes into ICO, GIF is the first largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
HEIC to ICO: 99,422 bytes in under a tenth of a second.
HEIF to ICO: 99,422 bytes in under a tenth of a second.
SVG to ICO: 99,422 bytes in under a tenth of a second.
TIFF to ICO: 99,422 bytes in under a tenth of a second.
WEBP to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about GIF to ICO
A GIF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A GIF to ICO run that passes 30 seconds is killed, and the ImageMagick process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
No text is read out of the GIF anywhere on this path. Converting a picture to ICO never turns it into words, whatever the picture happens to contain.
ImageMagick was not saturated at sixteen simultaneous conversions: it held 47.5 a second while latency moved from 0.22 s to 0.34 s, and the service memory did not move off 51 MB. That is the measured headroom behind GIF to ICO, not a marketing number.
One GIF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
GIF to ICO: what people ask
What actually converts my GIF file to ICO?
ImageMagick does it, 6.9.12, on a machine we rent and watch. Not a browser trick and not somebody else service: the GIF file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept afterwards.
How long does GIF to ICO take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 202,814 bytes of ICO. That is one real measurement on one real GIF file, not an average and not a promise about yours: a larger GIF takes longer, and past 30 seconds the run is stopped.
What do I lose going from GIF to ICO?
The one to know about first: Every frame of the GIF but the first. GIF can hold an animation and ICO cannot, so frame one is what comes back. The alternative would be one numbered ICO file per frame and no single answer to hand you.
Can I use the ICO as a favicon?
Yes, and it is the usual reason for this pair. ICO is the Windows icon container, and the size of the GIF you send decides what you get: anything over 256 pixels a side is scaled down to fit, anything under is left exactly as it is, so a 32 pixel GIF makes a 32 pixel icon. Send a square one if you can.
Is GIF to ICO free?
There is a free allowance every month, and one GIF to ICO conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse ICO.