HEIF is the container HEIC lives in, read here and not written. 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 HEIF file on the way to becoming a GIF one.
What runs when a HEIF file becomes GIF
ImageMagick 6.9.12, on a machine we rent and watch. Your HEIF file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept. HEIF 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 HEIF into the GIF
The pixel dimensions of your HEIF file. The GIF comes out the size the HEIF went in, because nothing on this path resizes anything.
What HEIF to GIF costs you
Colour depth. GIF holds 256 colours at most, so a photographic HEIF gets quantised and its gradients band.
Where HEIF and GIF files come from
HEIF. The container standard; HEIC is the name it goes by when the picture inside is HEVC. Read here and not written, because the write alias is not wired on the engine and pretending otherwise would hand you a file under the wrong name.
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 HEIF is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
HEIF to GIF, measured rather than promised
Running HEIF to GIF against the real engine with a real file, ImageMagick wrote 2,918 bytes of GIF in 0.1 seconds, machine otherwise idle. That is one HEIF file on one day and not an average, which is why the number is given together with the file that produced it.
The HEIF 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 HEIF file, sent somewhere else
Other outputs the probe measured out of a HEIF 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 HEIF witness file.
HEIF to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
HEIF to JPG: 1,666 bytes in under a tenth of a second, verified by a format signature.
HEIF to PNG: 1,403 bytes in under a tenth of a second, verified by a format signature.
HEIF to WEBP: 416 bytes in under a tenth of a second, verified by a format signature.
HEIF to TIFF: 90,264 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, HEIF is the number 18 largest output of the 19 measured. The witness files differ, so this ranks the probe run and not your document.
ICO to GIF: 3,799 bytes in 0.1 seconds.
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.
What we will not pretend about HEIF to GIF
A HEIF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A HEIF 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 HEIF 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 HEIF to GIF, not a marketing number.
One HEIF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
HEIF to GIF: what people ask
What actually converts my HEIF 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 HEIF file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept afterwards.
How long does HEIF to GIF take?
On the file the probe used, ImageMagick took 0.1 seconds and wrote 2,918 bytes of GIF. That is one real measurement on one real HEIF file, not an average and not a promise about yours: a larger HEIF takes longer, and past 30 seconds the run is stopped.
What do I lose going from HEIF to GIF?
The one to know about first: Colour depth. GIF holds 256 colours at most, so a photographic HEIF gets quantised and its gradients band.
Does the GIF come out the same size as the HEIF?
The same pixel size, yes: nothing on this path resizes anything, so the GIF is as wide and as tall as the HEIF was. The number of bytes is a different question and usually moves a long way, which is normally the reason for doing this at all.
Is HEIF to GIF free?
There is a free allowance every month, and one HEIF 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.