WebP is lossy or lossless depending on the encoder, with alpha and animation. 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 WEBP file on the way to becoming a JPG one.
What runs when a WEBP file becomes JPG
ImageMagick 6.9.12, on a machine we rent and watch. Your WEBP file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept. WEBP 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 WEBP into the JPG
The pixel dimensions of your WEBP file. The JPG comes out the size the WEBP went in, because nothing on this path resizes anything.
What WEBP to JPG costs you
Transparency. JPG has no alpha channel, so the engine composites your WEBP image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent WEBP logo comes back as a black rectangle.
Every frame of the WEBP but the first. WEBP 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 WEBP you sent. The artefacts appear once and never come out again, which is why converting the JPG back to WEBP afterwards restores nothing.
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 WEBP is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
WEBP to JPG, measured rather than promised
Running WEBP to JPG against the real engine with a real file, ImageMagick wrote 2,340 bytes of JPG in under a tenth of a second, machine otherwise idle. That is one WEBP file on one day and not an average, which is why the number is given together with the file that produced it.
The WEBP 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 WEBP file, sent somewhere else
Other outputs the probe measured out of a WEBP 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 WEBP witness file.
WEBP to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
WEBP to PNG: 4,003 bytes in under a tenth of a second, verified by a format signature.
WEBP to GIF: 4,744 bytes in 0.1 seconds, verified by a format signature.
WEBP to TIFF: 72,264 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, WEBP is the fourth largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
PDF to JPG: 5,782 bytes in 0.1 seconds.
GIF to JPG: 20,559 bytes in under a tenth of a second.
BMP to JPG: 1,568 bytes in under a tenth of a second.
TIFF to JPG: 1,568 bytes in under a tenth of a second.
AVIF to JPG: 1,705 bytes in under a tenth of a second.
What we will not pretend about WEBP to JPG
A WEBP file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A WEBP 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 WEBP 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 WEBP to JPG, not a marketing number.
One WEBP file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
WEBP to JPG: what people ask
What actually converts my WEBP 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 WEBP file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept afterwards.
How long does WEBP to JPG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 2,340 bytes of JPG. That is one real measurement on one real WEBP file, not an average and not a promise about yours: a larger WEBP takes longer, and past 30 seconds the run is stopped.
What do I lose going from WEBP to JPG?
The one to know about first: Transparency. JPG has no alpha channel, so the engine composites your WEBP image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent WEBP logo comes back as a black rectangle.
Why did my transparent WEBP 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 WEBP logo comes back as a black rectangle. If you need the transparency, send the WEBP to PNG or AVIF instead.
Is WEBP to JPG free?
There is a free allowance every month, and one WEBP 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.