When BMP to PNG turns BMP into PNG
BMP files from older Windows software are huge because they are barely compressed.
What BMP to PNG changes in the picture
The bitmap is redrawn as PNG, which is smaller and fine for screenshots and diagrams. A 24-bit BMP has no transparency to keep.
PNG will not invent detail. If the BMP is a photo, JPG is usually smaller still.
When BMP to PNG is the right page
BMP to PNG sits on this site because BMP and PNG are not interchangeable, even when they look similar in a file list. BMP files from older Windows software are huge because they are barely compressed. Machine tools, textbook CDs, and old Windows programs sometimes open only BMP. Use PNG when a logo must stay crisp or when the background must stay transparent.
The bitmap is redrawn as PNG, which is smaller and fine for screenshots and diagrams. A 24-bit BMP has no transparency to keep.
What you give up by leaving BMP
BMP is an uncompressed Windows bitmap. Files are large. You mostly meet it in older software. Transparency is lost, and the file grows a lot compared with PNG.
Keep the PNG. Send BMP only when the receiving software named it.
What you gain, and do not gain, with PNG
The download has to be PNG, because that is the format the next step named. Use PNG when a logo must stay crisp or when the background must stay transparent. Text in a screenshot stays sharper than it does in a heavy JPG.
PNG does not rescue a photo that was already compressed as JPG.
A test that matches the real file
Start from the question the other software asked. If it asked for PNG, BMP to PNG is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. BMP files from older Windows software are huge because they are barely compressed. The bitmap is redrawn as PNG, which is smaller and fine for screenshots and diagrams. A 24-bit BMP has no transparency to keep. The limit is the part people skip, and it is the part that decides whether the download is usable. PNG will not invent detail. If the BMP is a photo, JPG is usually smaller still. The file stays in your browser and is not uploaded. The limit is 100 MB. Keep the original until the download has been opened in the program that will use it. Check three things in the download: it opens, the BMP material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original BMP as well when you are allowed to. BMP to PNG is a convenience copy. It is not a new master. Photos fail uploads for three separate reasons: the format name, the pixel size, and the kilobyte size. BMP to PNG only does the BMP to PNG step. If the form also prints a pixel size or a 20 KB or 50 KB cap, do that step with the matching tool, starting from the original each time. A second lossy save will not recover detail.
Here is the decision for BMP to PNG. You start with BMP and you stop when a real program accepts PNG. Those old programs get a file they understand. Photographs are usually much larger as PNG than as JPG, with little visual gain. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
PNG will not invent detail. If the BMP is a photo, JPG is usually smaller still.
A related step, if you still need one, is PNG to BMP.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.