HEIF is the container HEIC lives in, read here and not written. JPEG is lossy compression tuned for human vision, and no alpha channel at all. ImageMagick sits between the two, and this page says exactly what happens to a HEIF file on the way to becoming a JPG one.
What runs when a HEIF file becomes JPG
ImageMagick 6.9.12, on a machine we rent and watch. Your HEIF file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept. HEIF to JPG 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 JPG
The pixel dimensions of your HEIF file. The JPG comes out the size the HEIF went in, because nothing on this path resizes anything.
What HEIF to JPG costs you
Transparency. JPG has no alpha channel, so the engine composites your HEIF image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent HEIF logo comes back as a black rectangle.
Every frame of the HEIF but the first. HEIF can hold an animation and JPG cannot, so frame one is what comes back. The alternative would be one numbered JPG file per frame and no single answer to hand you.
Exactness. JPG is a lossy encoding, so the file you get back approximates the HEIF you sent. The artefacts appear once and never come out again, which is why converting the JPG back to HEIF afterwards restores nothing.
Where HEIF and JPG 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 JPG
A JPG file is pixels, and pixels are expensive for a model to read. Handed this JPG, 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 JPG, measured rather than promised
Running HEIF to JPG against the real engine with a real file, ImageMagick wrote 1,666 bytes of JPG in under a tenth of a second, 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 JPG 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 JPG 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 JPG 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 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 GIF: 2,918 bytes in 0.1 seconds, 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 JPG, and what they measured
Among the published routes into JPG, HEIF is the seventh largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
JP2 to JPG: 1,568 bytes in under a tenth of a second.
HEIC to JPG: 1,666 bytes in under a tenth of a second.
PNG to JPG: 1,568 bytes in under a tenth of a second.
WEBP to JPG: 2,340 bytes in under a tenth of a second.
PDF to JPG: 5,782 bytes in 0.1 seconds.
What we will not pretend about HEIF to JPG
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 JPG 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 JPG 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 JPG, 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 JPG: what people ask
What actually converts my HEIF file to JPG?
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 JPG comes back, and neither file is kept afterwards.
How long does HEIF to JPG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 1,666 bytes of JPG. 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 JPG?
The one to know about first: Transparency. JPG has no alpha channel, so the engine composites your HEIF image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent HEIF logo comes back as a black rectangle.
Why did my transparent HEIF background turn white in the JPG?
Because JPG has no alpha channel and something has to sit behind the picture. The engine puts white there on purpose. Left to itself ImageMagick puts black, and a transparent HEIF logo comes back as a black rectangle. If you need the transparency, send the HEIF to PNG or WebP instead.
Is HEIF to JPG free?
There is a free allowance every month, and one HEIF to JPG 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 JPG.