Convert BMP to JXL

JPEG XL came out of the JPEG committee and was standardised as ISO/IEC 18181-1 in 2022. It compresses losslessly as well as lossily, handles up to 32 bits per channel, carries alpha and animation, and was explicitly aimed at being the format you keep an image in rather than the one you serve.

That is a good description of what somebody with a large uncompressed bitmap wants. A BMP is a master by accident — it is uncompressed because nothing writes the run-length modes the format defines, not because anyone chose fidelity — and moving it into something that keeps the pixels and stops costing three bytes each is a sensible instinct. The registry lists archival and photography as JPEG XL’s uses, and largeFiles as BMP’s known problem, so on paper the two were made for each other. The rest of this page is about the distance between that reading and what this converter can actually give you.

The encoder here produces lossy JPEG XL at quality 82, the same default every lossy target on this site uses. It does not write the lossless mode, and there is no setting on the page that switches to it.

So the instinct above lands on the wrong page. What you get is a small, modern, well-compressed image that is not the bitmap you started with, in a container most software still cannot open. That combination is worse than either problem on its own, and it is the reason this section is second rather than last. Saying it here rather than in a footnote is the point: a conversion that quietly did the lossy thing while the reader believed they were archiving would produce exactly the outcome an archive exists to prevent, and they would not find out for years.

PNG is lossless, it has been standardised since 1996, every browser and every editor reads it, and on the flat material that BMP files usually contain it compresses hard. A 960 by 640 photograph measured here came to 468 KB as a PNG against 1.84 MB as a BMP, and flat artwork does considerably better than that.

It is not as efficient as lossless JPEG XL would be. It is available, it is exact, and in thirty years it will still open — which is the only property an archival format is actually judged on. The bitmaps being converted here are themselves evidence of that: they survive because the format is simple and everything reads it, not because it was ever good. Choose PNG for the master and come back to a lossy format when you know where the image is going.

A large share of BMP files are pages rather than pictures — scans and faxes written by imaging software that had no other option. JPEG XL has nothing to offer them. A lossy encode of a page of type is exactly the wrong operation, and the container is one that no records system, no colleague and no regulator will accept.

For those, PDF is the deliverable and TIFF is what an archive specification usually names, both of which this site converts a bitmap into directly. If the words are what matters rather than the image of them, the BMP to TXT conversion reads the page with optical character recognition. Deciding which of those three you actually want takes less time than converting the folder twice.

The registry marks browser support for JXL as partial, next to the "all" it gives PNG, JPEG, GIF and WebP. That is not a temporary state of a new format; JPEG XL has had an uneven reception since it was standardised, and the situation has been unsettled for years rather than months.

The practical consequence is that a JXL is not a file to send anybody or to put on a page without an alternative. It is a file you keep for yourself, on machines whose software you control — which is a legitimate use, and a much narrower one than the format’s technical merits suggest. The registry marks the format’s status as niche and names patchy support as its known problem, and both of those are editorial judgements about adoption rather than about engineering. On the engineering, JPEG XL is the strongest still-image format in this registry.

The registry names GIMP and ImageMagick. Both read it properly and both are free, so the format is not locked away — but neither is what most people have in front of them when they double-click a file.

Operating system previewers, stock photo managers and several mainstream editors still show nothing. That is worth testing on your own machine before converting a folder, because a directory of files that will not preview is a genuinely annoying thing to discover a month later.

It gives clean, unprocessed pixels. A BMP has been through no lossy encoder, so nothing in it is an artefact of a previous compression, and that is the best possible input for any encoder — there is no noise to spend bits describing that was not in the original scene.

It withholds everything else. Eight bits per channel against the format’s 32, plain RGB with no wide-gamut information, no colour profile, no EXIF, no capture data — the registry records no metadata containers for BMP at all. JPEG XL’s headroom stays empty because there is nothing in the source to put in it, and the file will be read as sRGB by whatever opens it. If the bitmap came off a calibrated instrument or a colour-managed scanner, that assumption is worth checking rather than inheriting.

JPEG XL holds an alpha channel, and nothing on this path flattens the image or asks for a background colour, so the see-through areas of a 32-bit bitmap survive. The 24-bit files that make up most BMP output have no alpha in the first place.

JXL also supports animation, and BMP does not, so that capability has nothing to receive. It is worth stating only because it is one more line in the specification that a bitmap source cannot make use of, which is the running theme of converting from a format that predates all of them.

Very much less, because almost anything is very much less than three bytes per pixel. A 1920 by 1080 bitmap is 6.2 MB before a single decision is made about the picture, and a lossy JPEG XL of it will be a small fraction of that.

Against WebP and AVIF the comparison is much closer and depends on the image. If the file size is the thing you are optimising, convert the same bitmap to two or three targets and compare the actual byte counts — that takes a minute here and settles a question no general rule answers reliably. And if the three come out within a few per cent of each other, which they often do, the decision should be made on support rather than on bytes. On that axis JPEG XL is last of the three, and it is not close.

The bitmap is decoded by the browser and encoded by a WebAssembly build of the JPEG XL encoder in the same tab. Nothing is sent anywhere, there is no account, and the ceiling is 100 MB per file — about 33 megapixels of uncompressed bitmap.

Drop a folder and the files convert one after another, keep their names with the extension swapped, and come back as a ZIP. Given everything above, converting two or three files first and checking that your own software opens them is time better spent than converting the folder twice. Keep the bitmaps until that check has been done. This conversion is lossy and there is no way back from the JXL to the pixels the bitmap held, so deleting the originals is the one step in the process that cannot be undone.