BMP is raw uncompressed pixels, which is why the files are so large. GIF is a palette of at most 256 colours, all-or-nothing transparency, and animation. ImageMagick sits between the two, and this page says exactly what happens to a BMP file on the way to becoming a GIF one.
What runs when a BMP file becomes GIF
ImageMagick 6.9.12, on a machine we rent and watch. Your BMP file is uploaded once, ImageMagick runs once, the GIF comes back, and neither file is kept. BMP to GIF 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 GIF
The pixel dimensions of your BMP file. The GIF comes out the size the BMP went in, because nothing on this path resizes anything.
What BMP to GIF costs you
Colour depth. GIF holds 256 colours at most, so a photographic BMP gets quantised and its gradients band.
If a model is going to read the GIF
A GIF file is pixels, and pixels are expensive for a model to read. Handed this GIF, 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 GIF, measured rather than promised
Running BMP to GIF against the real engine with a real file, ImageMagick wrote 3,799 bytes of GIF in 0.1 seconds, 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 GIF 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 GIF 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 GIF 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 PNG: 810 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 TIFF: 72,264 bytes in under a tenth of a second, verified by a format signature.
Other ways into GIF, and what they measured
Among the published routes into GIF, BMP is the number 12 largest output of the 19 measured. The witness files differ, so this ranks the probe run and not your document.
FLV to GIF: 173,530 bytes in 0.1 seconds.
HEIC to GIF: 2,918 bytes in 0.1 seconds.
HEIF to GIF: 2,918 bytes in 0.1 seconds.
ICO to GIF: 3,799 bytes in 0.1 seconds.
MKV to GIF: 164,734 bytes in 0.1 seconds.
What we will not pretend about BMP to GIF
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 GIF 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 GIF 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 GIF, 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 GIF: what people ask
What actually converts my BMP file to GIF?
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 GIF comes back, and neither file is kept afterwards.
How long does BMP to GIF take?
On the file the probe used, ImageMagick took 0.1 seconds and wrote 3,799 bytes of GIF. 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 GIF?
The one to know about first: Colour depth. GIF holds 256 colours at most, so a photographic BMP gets quantised and its gradients band.
Can I make an animated GIF this way?
No. One BMP file goes in and one still GIF comes out. A BMP holds a single image, and nothing here assembles several files into a sequence: what you get is that image in a GIF wrapper, with the 256 colours GIF allows.
Is BMP to GIF free?
There is a free allowance every month, and one BMP to GIF 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 GIF.