What Is a BMP File and Why Is It So Big?

BMP stands for bitmap. Microsoft introduced it with early versions of Windows, and it stores an image as a plain grid of pixels with little or no compression. It was convenient for software that wanted to read and write pictures quickly, back when processing power was scarce.

A normal 24-bit BMP uses three bytes for every pixel. A 4000 by 3000 image has 12 million pixels, so the file is about 36 MB. The same picture as a JPG would be 2 to 4 MB, and as a PNG typically 10 to 20 MB.

Old Windows programs, some scanners and industrial or medical equipment, embedded devices, and the default screenshot of some legacy tools still produce BMP. Many modern web forms and apps reject the format, and it is a poor choice for sharing because of its size.

Use JPG for photos: the file will be about 5 to 10 percent of the BMP size. Use PNG for screenshots, drawings and anything with sharp text. PNG is lossless, so the pixels stay exactly as they were, and it still typically shrinks to a fraction of the original.

PNG loses nothing. JPG discards fine detail, but at quality 85 or higher the difference is hard to see. Keep the BMP if you need an untouched master copy for archiving, and share the converted version.