HEIC is what an iPhone writes unless you tell it not to: HEVC compression inside an ISO container, about half the size of the JPEG it replaced. 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 HEIC file on the way to becoming a BMP one.
What runs when a HEIC file becomes BMP
ImageMagick 6.9.12, on a machine we rent and watch. Your HEIC file is uploaded once, ImageMagick runs once, the BMP comes back, and neither file is kept. HEIC 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 HEIC into the BMP
The pixel dimensions of your HEIC file. The BMP comes out the size the HEIC went in, because nothing on this path resizes anything.
What HEIC to BMP costs you
Transparency. BMP has no alpha channel, so the engine composites your HEIC image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent HEIC logo comes back as a black rectangle.
Every page of the HEIC but the first. A HEIC file can hold several images in one container; a BMP file holds exactly one.
Nothing further, and that is the point worth making: BMP cannot undo what HEIC already threw away. The compression artefacts are baked into the pixels, BMP stores them faithfully, and the file usually comes out several times larger for exactly the same picture.
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 HEIC is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
HEIC to BMP, measured rather than promised
Running HEIC 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 HEIC file on one day and not an average, which is why the number is given together with the file that produced it.
The HEIC 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 HEIC file, sent somewhere else
Other outputs the probe measured out of a HEIC 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 HEIC witness file.
HEIC to PDF: 94,782 bytes in under a tenth of a second, verified by a format signature.
HEIC to JPG: 1,666 bytes in under a tenth of a second, verified by a format signature.
HEIC to PNG: 1,403 bytes in under a tenth of a second, verified by a format signature.
HEIC to WEBP: 416 bytes in under a tenth of a second, verified by a format signature.
HEIC to GIF: 2,918 bytes in 0.1 seconds, verified by a format signature.
HEIC to TIFF: 90,264 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, HEIC is the fourth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
HEIF to BMP: 72,138 bytes in under a tenth of a second.
ICO to BMP: 72,138 bytes in under a tenth of a second.
SVG to BMP: 72,138 bytes in under a tenth of a second.
TIFF to BMP: 72,138 bytes in under a tenth of a second.
WEBP to BMP: 72,138 bytes in under a tenth of a second.
What we will not pretend about HEIC to BMP
A HEIC file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A HEIC 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.
Until the end of August this machine could not read a HEIC at all, and reported the opposite. ImageMagick declared HEIC readable, but libheif carried only its AV1 plugins, which serve AVIF; the HEVC decoder was absent, and a HEIC decoded quietly as something else. The plugin was installed on the 30th, and that is the whole reason this pair exists.
The HEIC to BMP proof used a file genuinely encoded in HEVC, and that detail is not pedantry: ImageMagick refuses to write HEIC and hands you a PNG under that name if you ask it to, so a HEIC forged for the test would have made a false result look like a true one. The BMP it produced was 72,138 bytes, recognised by a weak signature.
No text is read out of the HEIC 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 HEIC to BMP, not a marketing number.
One HEIC file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
HEIC to BMP: what people ask
What actually converts my HEIC 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 HEIC file is uploaded once, ImageMagick runs once, the BMP comes back, and neither file is kept afterwards.
How long does HEIC 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 HEIC file, not an average and not a promise about yours: a larger HEIC takes longer, and past 30 seconds the run is stopped.
What do I lose going from HEIC to BMP?
The one to know about first: Transparency. BMP has no alpha channel, so the engine composites your HEIC image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent HEIC logo comes back as a black rectangle.
Why did my transparent HEIC 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 HEIC logo comes back as a black rectangle. If you need the transparency, send the HEIC to PNG or WebP instead.
Is HEIC to BMP free?
There is a free allowance every month, and one HEIC 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.