WebP is lossy or lossless depending on the encoder, with alpha and animation. 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 WEBP file on the way to becoming a PNG one.
What runs when a WEBP file becomes PNG
ImageMagick 6.9.12, on a machine we rent and watch. Your WEBP file is uploaded once, ImageMagick runs once, the PNG comes back, and neither file is kept. WEBP 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 WEBP into the PNG
The pixel dimensions of your WEBP file. The PNG comes out the size the WEBP went in, because nothing on this path resizes anything.
Transparency. WEBP and PNG both carry a real alpha channel, so a cut-out sent as WEBP is still a cut-out as PNG.
What WEBP to PNG costs you
Every frame of the WEBP but the first. WEBP can hold an animation and PNG cannot, so frame one is what comes back. The alternative would be one numbered PNG file per frame and no single answer to hand you.
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 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 PNG, measured rather than promised
Running WEBP to PNG against the real engine with a real file, ImageMagick wrote 4,003 bytes of PNG 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 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 WEBP file, sent somewhere else
Other outputs the probe measured out of a WEBP 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 WEBP witness file.
WEBP to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
WEBP to JPG: 2,340 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 PNG, and what they measured
Among the published routes into PNG, WEBP is the second largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
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.
PDF to PNG: 1,294 bytes in 0.1 seconds.
What we will not pretend about WEBP to PNG
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 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 WEBP 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 WEBP to PNG, 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 PNG: what people ask
What actually converts my WEBP 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 WEBP file is uploaded once, ImageMagick runs once, the PNG comes back, and neither file is kept afterwards.
How long does WEBP to PNG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 4,003 bytes of PNG. 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 PNG?
The one to know about first: Every frame of the WEBP but the first. WEBP can hold an animation and PNG cannot, so frame one is what comes back. The alternative would be one numbered PNG file per frame and no single answer to hand you.
My WEBP is animated. Will the PNG be?
No. A WEBP can hold a whole sequence and a PNG holds one image, so what comes back is the first frame. The alternative would be one numbered PNG per frame, and there is no single file to hand you at the end of that.
Is WEBP to PNG free?
There is a free allowance every month, and one WEBP 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.