BMP is raw uncompressed pixels, which is why the files are so large. ICO is a Windows icon, capped at 256 pixels a side by the file structure itself. ImageMagick sits between the two, and this page says exactly what happens to a BMP file on the way to becoming an ICO one.
What runs when a BMP file becomes ICO
ImageMagick 6.9.12, on a machine we rent and watch. Your BMP file is uploaded once, ImageMagick runs once, the ICO comes back, and neither file is kept. BMP to ICO 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 ICO
Every pixel value. BMP and ICO are both lossless, so the ICO file adds no compression artefacts that the BMP did not already have.
What BMP to ICO costs you
Any BMP bigger than 256 pixels a side. That is not a setting, it is the ICO file structure: width and height each live in a single byte. The engine scales larger sources down, and only down, so a phone photo becomes a 256 pixel icon.
If a model is going to read the ICO
An ICO file is pixels, and pixels are expensive for a model to read. Handed this ICO, 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 ICO, measured rather than promised
Running BMP to ICO against the real engine with a real file, ImageMagick wrote 99,422 bytes of ICO 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 ICO 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 ICO 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 ICO 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 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 ICO, and what they measured
Among the published routes into ICO, BMP is the fifth largest output of the 10 measured. The witness files differ, so this ranks the probe run and not your document.
GIF to ICO: 202,814 bytes in under a tenth of a second.
HEIC to ICO: 99,422 bytes in under a tenth of a second.
HEIF to ICO: 99,422 bytes in under a tenth of a second.
SVG to ICO: 99,422 bytes in under a tenth of a second.
TIFF to ICO: 99,422 bytes in under a tenth of a second.
What we will not pretend about BMP to ICO
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 ICO 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 ICO 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 ICO, 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 ICO: what people ask
What actually converts my BMP file to ICO?
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 ICO comes back, and neither file is kept afterwards.
How long does BMP to ICO take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 99,422 bytes of ICO. 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 ICO?
The one to know about first: Any BMP bigger than 256 pixels a side. That is not a setting, it is the ICO file structure: width and height each live in a single byte. The engine scales larger sources down, and only down, so a phone photo becomes a 256 pixel icon.
Can I use the ICO as a favicon?
Yes, and it is the usual reason for this pair. ICO is the Windows icon container, and the size of the BMP you send decides what you get: anything over 256 pixels a side is scaled down to fit, anything under is left exactly as it is, so a 32 pixel BMP makes a 32 pixel icon. Send a square one if you can.
Is BMP to ICO free?
There is a free allowance every month, and one BMP to ICO 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 ICO.