BMP to JPG

BMP stores every pixel in full, with no compression worth the name, which is why the files are so startlingly large. That is a deliberate design from an era when decoding speed mattered more than disk space, and it survives today mainly in Windows utilities, older scanner and camera drivers, screen-capture tools, and industrial or medical equipment that was specified decades ago. The moment you need to email one, put it on a website, or hand it to almost any modern application, the size becomes the problem. Converting to JPG typically cuts it by 90% or more with no visible difference at a sensible quality setting. It also solves a compatibility issue that catches people out: while BMP is trivially readable on Windows, support on macOS, iOS and Android is patchier than you would expect for such an old format.

A 90–97% reduction is normal, and it is not a sign that something has gone wrong. You are moving from a format that stores each pixel literally to one that models what human vision actually notices. If you need the reduction without any loss at all, convert to PNG instead — still a large saving over BMP, typically 40–70%, and lossless.

Most tools here hand very large files to a server for processing, but BMP is deliberately excluded from that: the server-side image library has no BMP decoder, so an offloaded bitmap would fail rather than convert. Your browser handles BMP perfectly well, so BMP conversions stay on your machine no matter how big the file is. Given how large bitmaps get, this is also the faster path — there is no upload to wait through.

Most BMP files in circulation are 24-bit with no transparency, so there is usually nothing to flatten. Some 32-bit BMPs do carry an alpha channel; because JPG cannot store one, any transparent area is filled with the background colour you choose. Very old 8-bit or 1-bit bitmaps convert fine, though a 1-bit black-and-white scan is often better served by PNG, which handles hard-edged monochrome more efficiently than JPG.

BMP is Microsoft's original Windows bitmap, and it is usually stored with no compression at all — every pixel written out in full. A 12-megapixel photo as a 24-bit BMP occupies about 36 MB, against roughly 3 MB as a JPG. It still appears in Windows utilities, older scanner drivers, medical and industrial equipment and some embedded systems, which is why converting out of it is a common need even though almost nobody chooses it deliberately.

JPG (also written JPEG) has been the default photograph format since 1992. It uses lossy compression tuned for the way human vision works — discarding fine colour detail the eye barely registers — which is why a photo saved as JPG can be a tenth the size of the same image as PNG. The trade-off is that it has no transparency channel and that every re-save degrades the image slightly, so it suits finished photographs rather than working files.