PNG is lossless compression with a full 8-bit alpha channel. 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 PNG file on the way to becoming a JPG one.
What runs when a PNG file becomes JPG
ImageMagick 6.9.12, on a machine we rent and watch. Your PNG file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept. PNG 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 PNG into the JPG
The pixel dimensions of your PNG file. The JPG comes out the size the PNG went in, because nothing on this path resizes anything.
What PNG to JPG costs you
Transparency. JPG has no alpha channel, so the engine composites your PNG image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent PNG logo comes back as a black rectangle.
Exactness. JPG is a lossy encoding, so the file you get back approximates the PNG you sent. The artefacts appear once and never come out again, which is why converting the JPG back to PNG 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 PNG is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
PNG to JPG, measured rather than promised
Running PNG to JPG against the real engine with a real file, ImageMagick wrote 1,568 bytes of JPG in under a tenth of a second, machine otherwise idle. That is one PNG file on one day and not an average, which is why the number is given together with the file that produced it.
The PNG 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 PNG file, sent somewhere else
Other outputs the probe measured out of a PNG 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 PNG witness file.
PNG to PDF: 2,536 bytes in under a tenth of a second, verified by a format signature.
PNG to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
PNG to GIF: 3,512 bytes in 0.1 seconds, verified by a format signature.
PNG to TIFF: 1,466 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, PNG is the eighth largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
WEBP to JPG: 2,340 bytes in under a tenth of a second.
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.
What we will not pretend about PNG to JPG
A PNG file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A PNG 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 PNG 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 PNG to JPG, not a marketing number.
One PNG file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
PNG to JPG: what people ask
What actually converts my PNG 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 PNG file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept afterwards.
How long does PNG to JPG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 1,568 bytes of JPG. That is one real measurement on one real PNG file, not an average and not a promise about yours: a larger PNG takes longer, and past 30 seconds the run is stopped.
What do I lose going from PNG to JPG?
The one to know about first: Transparency. JPG has no alpha channel, so the engine composites your PNG image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent PNG logo comes back as a black rectangle.
Why did my transparent PNG 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 PNG logo comes back as a black rectangle. If you need the transparency, send the PNG to WebP or AVIF instead.
Is PNG to JPG free?
There is a free allowance every month, and one PNG 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.