JPEG is lossy compression tuned for human vision, and no alpha channel at all. PNG is lossless compression with a full 8-bit alpha channel. ImageMagick sits between the two, and this page says exactly what happens to a JPG file on the way to becoming a PNG one.
What runs when a JPG file becomes PNG
ImageMagick 6.9.12, on a machine we rent and watch. Your JPG file is uploaded once, ImageMagick runs once, the PNG comes back, and neither file is kept. JPG to PNG 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 PNG
The pixel dimensions of your JPG file. The PNG comes out the size the JPG went in, because nothing on this path resizes anything.
What JPG to PNG costs you
Nothing further, and that is the point worth making: PNG cannot undo what JPG already threw away. The compression artefacts are baked into the pixels, PNG stores them faithfully, and the file usually comes out several times larger for exactly the same picture.
If a model is going to read the PNG
A PNG file is pixels, and pixels are expensive for a model to read. Handed this PNG, 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 PNG, measured rather than promised
Running JPG to PNG against the real engine with a real file, ImageMagick wrote 805 bytes of PNG in under a tenth of a second, 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 PNG 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 PNG 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 PNG 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 WEBP: 350 bytes in under a tenth of a second, verified by a format signature.
JPG to GIF: 3,487 bytes in 0.1 seconds, 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 PNG, and what they measured
Among the published routes into PNG, JPG is the number 12 largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
WEBP to PNG: 4,003 bytes in under a tenth of a second.
SVG to PNG: 1,268 bytes in under a tenth of a second.
GIF to PNG: 3,894 bytes in under a tenth of a second.
BMP to PNG: 810 bytes in under a tenth of a second.
TIFF to PNG: 815 bytes in under a tenth of a second.
What we will not pretend about JPG to PNG
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 PNG 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 PNG 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 PNG, 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 PNG: what people ask
What actually converts my JPG file to PNG?
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 PNG comes back, and neither file is kept afterwards.
How long does JPG to PNG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 805 bytes of PNG. 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 PNG?
The one to know about first: Nothing further, and that is the point worth making: PNG cannot undo what JPG already threw away. The compression artefacts are baked into the pixels, PNG stores them faithfully, and the file usually comes out several times larger for exactly the same picture.
Why is the PNG bigger than the JPG it came from?
Because JPG is a lossy encoding and PNG is not. The JPG reached its size by throwing detail away; PNG stores every pixel it is handed, compression artefacts included, so the same picture comes back several times larger. Nothing was added and no quality was recovered.
Is JPG to PNG free?
There is a free allowance every month, and one JPG to PNG 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 PNG.