JPEG is lossy compression tuned for human vision, and no alpha channel at all. 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 a JPG file on the way to becoming a GIF one.
What runs when a JPG file becomes GIF
ImageMagick 6.9.12, on a machine we rent and watch. Your JPG file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept. JPG 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 JPG into the GIF
The pixel dimensions of your JPG file. The GIF comes out the size the JPG went in, because nothing on this path resizes anything.
What JPG to GIF costs you
Colour depth. GIF holds 256 colours at most, so a photographic JPG 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 JPG is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
JPG to GIF, measured rather than promised
Running JPG to GIF against the real engine with a real file, ImageMagick wrote 3,487 bytes of GIF in 0.1 seconds, machine otherwise idle. That is one JPG file on one day and not an average, which is why the number is given together with the file that produced it.
The JPG 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 JPG file, sent somewhere else
Other outputs the probe measured out of a JPG 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 JPG witness file.
JPG to PDF: 4,232 bytes in under a tenth of a second, verified by a format signature.
JPG to PNG: 805 bytes in under a tenth of a second, verified by a format signature.
JPG to WEBP: 350 bytes in under a tenth of a second, verified by a format signature.
JPG to TIFF: 72,264 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, JPG is the number 16 largest output of the 19 measured. The witness files differ, so this ranks the probe run and not your document.
3GP to GIF: 164,004 bytes in 0.1 seconds.
AVI to GIF: 173,720 bytes in 0.2 seconds.
BMP to GIF: 3,799 bytes in 0.1 seconds.
FLV to GIF: 173,530 bytes in 0.1 seconds.
HEIC to GIF: 2,918 bytes in 0.1 seconds.
What we will not pretend about JPG to GIF
A JPG file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A JPG 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 JPG 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 JPG to GIF, not a marketing number.
One JPG file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
JPG to GIF: what people ask
What actually converts my JPG 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 JPG file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept afterwards.
How long does JPG to GIF take?
On the file the probe used, ImageMagick took 0.1 seconds and wrote 3,487 bytes of GIF. That is one real measurement on one real JPG file, not an average and not a promise about yours: a larger JPG takes longer, and past 30 seconds the run is stopped.
What do I lose going from JPG to GIF?
The one to know about first: Colour depth. GIF holds 256 colours at most, so a photographic JPG gets quantised and its gradients band.
Can I make an animated GIF this way?
No. One JPG file goes in and one still GIF comes out. A JPG 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 JPG to GIF free?
There is a free allowance every month, and one JPG 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.