ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. GIF is a palette of at most 256 colours, all-or-nothing transparency, and animation. ImageMagick sits between the two, and this page says exactly what happens to an ICO file on the way to becoming a GIF one.
What runs when an ICO file becomes GIF
ImageMagick 6.9.12, on a machine we rent and watch. Your ICO file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept. ICO to GIF 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 survives from the ICO into the GIF
The pixel dimensions of your ICO file. The GIF comes out the size the ICO went in, because nothing on this path resizes anything.
What ICO to GIF costs you
Colour depth. GIF holds 256 colours at most, so a photographic ICO gets quantised and its gradients band.
If a model is going to read the GIF
A GIF file is pixels, and pixels are expensive for a model to read. Handed this GIF, an assistant spends its budget looking at the picture rather than reading it, and it will paraphrase what it thinks it saw. If your ICO is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
ICO to GIF, measured rather than promised
Running ICO to GIF against the real engine with a real file, ImageMagick wrote 3,799 bytes of GIF in 0.1 seconds, machine otherwise idle. That is one ICO file on one day and not an average, which is why the number is given together with the file that produced it.
The ICO to GIF 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 GIF file has anything inside it.
The same ICO file, sent somewhere else
Other outputs the probe measured out of an ICO file, so the cost of choosing GIF can be read against something. Sizes do not compare across engines, because each family was probed with its own ICO witness file.
ICO to PDF: 95,344 bytes in under a tenth of a second, verified by a format signature.
ICO to JPG: 1,568 bytes in under a tenth of a second, verified by a format signature.
ICO to PNG: 810 bytes in under a tenth of a second, verified by a format signature.
ICO to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
ICO to TIFF: 96,278 bytes in under a tenth of a second, verified by a format signature.
Other ways into GIF, and what they measured
Among the published routes into GIF, ICO is the number 13 largest output of the 19 measured. The witness files differ, so this ranks the probe run and not your document.
MKV to GIF: 164,734 bytes in 0.1 seconds.
MOV to GIF: 164,004 bytes in 0.1 seconds.
MPG to GIF: 184,860 bytes in 0.1 seconds.
SVG to GIF: 1,232 bytes in 0.1 seconds.
TIFF to GIF: 3,512 bytes in 0.1 seconds.
What we will not pretend about ICO to GIF
An ICO file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An ICO to GIF 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 ICO anywhere on this path. Converting a picture to GIF 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 ICO to GIF, not a marketing number.
One ICO file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
ICO to GIF: what people ask
What actually converts my ICO file to GIF?
ImageMagick does it, 6.9.12, on a machine we rent and watch. Not a browser trick and not somebody else service: the ICO file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept afterwards.
How long does ICO to GIF take?
On the file the probe used, ImageMagick took 0.1 seconds and wrote 3,799 bytes of GIF. That is one real measurement on one real ICO file, not an average and not a promise about yours: a larger ICO takes longer, and past 30 seconds the run is stopped.
What do I lose going from ICO to GIF?
The one to know about first: Colour depth. GIF holds 256 colours at most, so a photographic ICO gets quantised and its gradients band.
Can I make an animated GIF this way?
No. One ICO file goes in and one still GIF comes out. An ICO holds a single image, and nothing here assembles several files into a sequence: what you get is that image in a GIF wrapper, with the 256 colours GIF allows.
Is ICO to GIF free?
There is a free allowance every month, and one ICO to GIF 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 GIF.