ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. 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 an ICO file on the way to becoming a BMP one.
What runs when an ICO file becomes BMP
ImageMagick 6.9.12, on a machine we rent and watch. Your ICO file is uploaded once, ImageMagick runs once, the BMP comes back, and neither file is kept. ICO 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 ICO into the BMP
The pixel dimensions of your ICO file. The BMP comes out the size the ICO went in, because nothing on this path resizes anything.
Every pixel value. ICO and BMP are both lossless, so the BMP file adds no compression artefacts that the ICO did not already have.
What ICO to BMP costs you
Transparency. BMP has no alpha channel, so the engine composites your ICO image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent ICO logo comes back as a black rectangle.
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 ICO is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
ICO to BMP, measured rather than promised
Running ICO 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 ICO file on one day and not an average, which is why the number is given together with the file that produced it.
The ICO 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 ICO file, sent somewhere else
Other outputs the probe measured out of an ICO 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 ICO witness file.
ICO to PDF: 95,344 bytes in under a tenth of a second, verified by a format signature.
ICO to JPG: 1,568 bytes in under a tenth of a second, verified by a format signature.
ICO to PNG: 810 bytes in under a tenth of a second, verified by a format signature.
ICO to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
ICO to GIF: 3,799 bytes in 0.1 seconds, verified by a format signature.
ICO to TIFF: 96,278 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, ICO is the sixth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
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.
PDF to BMP: 313,138 bytes in 0.1 seconds.
JPG to BMP: 72,138 bytes in under a tenth of a second.
What we will not pretend about ICO to BMP
An ICO file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An ICO 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 ICO 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 ICO to BMP, not a marketing number.
One ICO file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
ICO to BMP: what people ask
What actually converts my ICO 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 ICO file is uploaded once, ImageMagick runs once, the BMP comes back, and neither file is kept afterwards.
How long does ICO 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 ICO file, not an average and not a promise about yours: a larger ICO takes longer, and past 30 seconds the run is stopped.
What do I lose going from ICO to BMP?
The one to know about first: Transparency. BMP has no alpha channel, so the engine composites your ICO image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent ICO logo comes back as a black rectangle.
Why did my transparent ICO 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 ICO logo comes back as a black rectangle. If you need the transparency, send the ICO to PNG or WebP instead.
Is ICO to BMP free?
There is a free allowance every month, and one ICO 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.