What happens if I turn a JPG into a BMP and then back into a JPG?

The image ends up worse than your first JPG. Moving to BMP adds no new loss, but when you go back to JPG the compression is applied a second time and the degradation accumulates. JPG's loss is cumulative; every re-save adds new artefacts on top of the previous ones. If you need to edit an image and republish it, try to make as few JPG saves as possible and use a lossless format for the intermediate steps.

Isn't BMP lossless? Then why don't I gain quality?

BMP really is lossless, but losslessness means preserving data, not improving it. When you convert to BMP, the degradation already present in the JPG is preserved in full; the ringing, blocking and softening are carried over exactly as they were. In other words, BMP gives back precisely what you put in, and nothing more. A format being lossless does not mean it will retroactively repair the content you put inside it.

Should I convert JPG to BMP for printing?

No, this contributes nothing to print quality. The formats expected in print workflows are usually TIFF or PDF; BMP is not a format used in prepress and it does not support features critical for printing such as the CMYK colour space and embedded colour profiles. What actually determines the outcome for an image going to print is the resolution and original quality of the source file. Turning a JPG into a BMP makes the file bigger but makes no difference at all on paper.

How can I be sure a program actually wants BMP?

Checking the program's documentation or the file type list on its import screen is the most reliable way. Supported extensions are usually listed explicitly. On older devices it is stated in the manual along with the bit depth; 24-bit BMP, for example. That detail matters because BMP has different variants, and a device may be expecting a specific variant rather than a generic BMP. If you are unsure, trying a small test file is the quickest method.

What Is Converting JPG to BMP Actually Good For?

The question itself is a little odd

"What is converting JPG to BMP good for?" is one of those questions among format conversions that should be asked least often but is answered wrongly most often. Because most conversions have an obvious benefit — the file gets smaller, compatibility improves, transparency is preserved. In this conversion, most of the benefits run in the opposite direction.

This article honestly explains what the conversion actually does and when it makes sense.

What it does not give you

Let us first eliminate the common expectations one by one, because they are the main source of demand for this conversion.

It does not improve quality. JPG is a lossy format. When your file was first saved, the compression algorithm calculated and discarded information the human eye barely notices. That operation is irreversible. When you move to BMP that data does not come back; only its current state is stored uncompressed.

Think of it this way: BMP is a lossless container, but a container being lossless does not repair its contents. Whatever you put in, it gives back exactly.

It does not improve sharpness. The ringing, blocking and softening visible in the JPG stay exactly where they were after the conversion. Zoom in and you will see no difference at all — because there is none.

It gives no printing advantage. The formats expected in print workflows are usually TIFF or PDF. BMP is not a format used in prepress; it supports neither the CMYK colour space nor embedded colour profiles. What determines the outcome in print is the resolution and original quality of the source file, not the name of the container format.

It gives no archival advantage. If you want lossless archiving, PNG does the same job and takes up dramatically less space. There is no technical justification whatsoever for choosing BMP to store an image long term.

What it costs you

Disk space, and plenty of it. BMP typically stores pixel data with no compression at all. Your JPG file was small because of aggressive compression; BMP removes that compression entirely and writes every pixel directly. File size is now purely the product of resolution and bit depth, with no relation to content.

Portability. The resulting file may not fit into an email attachment, cloud uploads take longer, and messaging apps will most likely refuse it.

Platform acceptance. Social media, content management systems and most web platforms reject BMP uploads. Browser behaviour with BMP is inconsistent too. So after the conversion your file can be used in fewer places.

So what is it actually good for?

After all that negative accounting, there is one clear answer: compatibility.

The only reason BMP survives today is that its file structure is extremely simple. A short header followed by a raw array of pixels. That means you do not need a complex decoder to read a BMP. Even a very constrained processor can read a few numbers from the header and push the pixels straight to the screen.

That property is still valuable in these contexts:

Embedded systems. Touch panels, controller displays and device interfaces run with limited memory and processing power; they may not carry the code needed to decode JPG or PNG.
Industrial machinery. Production line equipment is long-lived and its software is rarely updated. An interface written years ago may still expect BMP.
Legacy Windows software. Desktop programs that have not been updated sometimes offer BMP as the only import option.
Old toolchains. A game engine's asset importer, a CAD plugin or a legacy automation script may not recognise any other format.
Device splash screens. Some hardware accepts its own display resources only as BMP.

In the situations on this list, the conversion is not merely useful but mandatory. In every scenario outside the list, it is probably unnecessary.

Things to watch if you are going to convert

If you genuinely need BMP, a few practical points:

Find out the bit depth. BMP is not a single thing; it has variants such as 24-bit, 32-bit and indexed 8-bit. An old device may expect a specific variant and reject a generic BMP. Look for this in the documentation.

Check the resolution constraint. Embedded systems usually expect a fixed screen size. A file of the wrong dimensions will not be accepted or will display incorrectly.

Try to find the source. If the JPG you have is the output of an edit, going from a lossless source produces a cleaner result than finding the original file and producing the BMP directly from there. You do not have to carry over JPG's flaws.

Do not delete the JPG. Your BMP file contains no extra information, it just takes up extra space. Let your master copy remain the JPG; treat the BMP as a temporary output produced for a job, and delete it when the job is done.

In summary

Converting from JPG to BMP is not a quality operation, it is a compatibility operation. Your image does not improve, your file grows, and it is accepted on fewer platforms. On the other hand, if there is an old program or device that expects BMP, the conversion solves the problem and there is no other way. The decision criterion fits in one sentence: if the target system accepts any other format, don't.