WebP is lossy or lossless depending on the encoder, with alpha and animation. BMP is raw uncompressed pixels, which is why the files are so large. ImageMagick sits between the two, and this page says exactly what happens to a WEBP file on the way to becoming a BMP one.
What runs when a WEBP file becomes BMP
ImageMagick 6.9.12, on a machine we rent and watch. Your WEBP file is uploaded once, ImageMagick runs once, the BMP comes back, and neither file is kept. WEBP to BMP 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 BMP
The pixel dimensions of your WEBP file. The BMP comes out the size the WEBP went in, because nothing on this path resizes anything.
What WEBP to BMP costs you
Transparency. BMP 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 BMP cannot, so frame one is what comes back. The alternative would be one numbered BMP file per frame and no single answer to hand you.
If a model is going to read the BMP
A BMP file is pixels, and pixels are expensive for a model to read. Handed this BMP, 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 BMP, measured rather than promised
Running WEBP to BMP against the real engine with a real file, ImageMagick wrote 72,138 bytes of BMP 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 BMP verdict was reached by a weak signature, meaning one or two bytes only, which is real evidence but leaves room for coincidence. 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 BMP 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 BMP 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 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 BMP, and what they measured
Among the published routes into BMP, WEBP is the ninth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
PDF to BMP: 313,138 bytes in 0.1 seconds.
JPG to BMP: 72,138 bytes in under a tenth of a second.
PNG to BMP: 72,138 bytes in under a tenth of a second.
GIF to BMP: 16,984 bytes in under a tenth of a second.
HEIC to BMP: 72,138 bytes in under a tenth of a second.
What we will not pretend about WEBP to BMP
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 BMP 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 BMP 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 BMP, 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 BMP: what people ask
What actually converts my WEBP file to BMP?
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 BMP comes back, and neither file is kept afterwards.
How long does WEBP to BMP take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 72,138 bytes of BMP. 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 BMP?
The one to know about first: Transparency. BMP 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 BMP?
Because BMP 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 BMP free?
There is a free allowance every month, and one WEBP to BMP 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 BMP.