What is a BMP file?
BMP, short for bitmap, is the picture format Microsoft shipped with Windows in 1987 and never really replaced. It stores the color of every single pixel one after another with no compression at all, preceded by a small header that says how wide the image is, how tall, and how many bits each pixel uses. That simplicity is the whole point: any program can read a BMP with a few lines of code, which is why scanners, screen-capture utilities, microscopes, cash registers, and industrial equipment still produce them.
The cost is size. A 12-megapixel photo is about 3 MB as a JPG and about 36 MB as a 24-bit BMP, because nothing is squeezed. There is an optional run-length encoding mode for 4 and 8-bit images, but almost nothing uses it, and the format has no real transparency support: a 32-bit BMP can carry an alpha channel, but so many programs ignore it that you cannot rely on it.
Where BMP still turns up
Old software and hardware that was never updated, Windows utilities that save screenshots, icon and cursor editors, some game modding pipelines, and equipment that writes images straight from a sensor. Windows Paint still offers BMP in its save dialog, which is how most people end up with one by accident.
It is also the format inside other files: an ICO icon is usually a small stack of BMP or PNG images, and some fonts and resource files embed bitmaps the same way.
What to convert a BMP to
PNG for anything with text, flat color, or sharp edges: it is lossless like BMP, so nothing changes visually, and the file usually shrinks by 60 to 90 percent because PNG actually compresses. JPG for photographs, where a quality of 90 gives a file a fraction of the size with no visible difference. WebP if the picture is headed for a web page and you want it smaller still.
Converting away from BMP never loses detail when you pick PNG, because both store the exact pixels. The only thing you lose is the wasted space.
Frequently asked questions
Why is my BMP file so large?
Because it is uncompressed: width times height times three bytes, plus a small header. A 1920 by 1080 screenshot is about 6 MB as a BMP and about 300 KB as a PNG of the same picture, pixel for pixel identical.
Does BMP support transparency?
Only in a 32-bit variant that many programs ignore, so in practice no. If you need transparency, use PNG.
Is BMP better quality than JPG?
It is lossless, so it has no compression artifacts, but it holds no more detail than the picture that went into it. A BMP made from a JPG still has the JPG artifacts, just stored in a much bigger file.
How do I open a BMP file?
Every operating system and browser already does. If something refuses it, convert it to PNG first.