BMP is raw uncompressed pixels, which is why the files are so large. 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 BMP file on the way to becoming a PNG one.
What runs when a BMP file becomes PNG
ImageMagick 6.9.12, on a machine we rent and watch. Your BMP file is uploaded once, ImageMagick runs once, the PNG comes back, and neither file is kept. BMP 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 BMP into the PNG
The pixel dimensions of your BMP file. The PNG comes out the size the BMP went in, because nothing on this path resizes anything.
Every pixel value. BMP and PNG are both lossless, so the PNG file adds no compression artefacts that the BMP did not already have.
What BMP to PNG costs you
Nothing measurable, which is rare enough to say plainly. BMP and PNG carry the same kind of content with the same fidelity, so this pair changes the container and not the picture. What does change is the size, and often by a lot.
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 BMP is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
BMP to PNG, measured rather than promised
Running BMP to PNG against the real engine with a real file, ImageMagick wrote 810 bytes of PNG in under a tenth of a second, machine otherwise idle. That is one BMP file on one day and not an average, which is why the number is given together with the file that produced it.
The BMP 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 BMP file, sent somewhere else
Other outputs the probe measured out of a BMP 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 BMP witness file.
BMP to PDF: 118,745 bytes in under a tenth of a second, verified by a format signature.
BMP to JPG: 1,568 bytes in under a tenth of a second, verified by a format signature.
BMP to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
BMP to GIF: 3,799 bytes in 0.1 seconds, verified by a format signature.
BMP 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, BMP is the tenth largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
TIFF to PNG: 815 bytes in under a tenth of a second.
PDF to PNG: 1,294 bytes in 0.1 seconds.
AVIF to PNG: 4,453 bytes in under a tenth of a second.
PSD to PNG: 689 bytes in under a tenth of a second.
ICO to PNG: 810 bytes in under a tenth of a second.
What we will not pretend about BMP to PNG
A BMP file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A BMP 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 BMP 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 BMP to PNG, not a marketing number.
One BMP file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
BMP to PNG: what people ask
What actually converts my BMP 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 BMP file is uploaded once, ImageMagick runs once, the PNG comes back, and neither file is kept afterwards.
How long does BMP to PNG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 810 bytes of PNG. That is one real measurement on one real BMP file, not an average and not a promise about yours: a larger BMP takes longer, and past 30 seconds the run is stopped.
What do I lose going from BMP to PNG?
The one to know about first: Nothing measurable, which is rare enough to say plainly. BMP and PNG carry the same kind of content with the same fidelity, so this pair changes the container and not the picture. What does change is the size, and often by a lot.
Does the PNG come out the same size as the BMP?
The same pixel size, yes: nothing on this path resizes anything, so the PNG is as wide and as tall as the BMP was. The number of bytes is a different question and usually moves a long way, which is normally the reason for doing this at all.
Is BMP to PNG free?
There is a free allowance every month, and one BMP 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.