Why BMP files are enormous — and the two-minute fix

2026-09-11

An old program exports "images" and each one is 8 MB. A colleague sends a screenshot that will not fit in an email. The files end in .bmp, and they are enormous out of all proportion to what they show.

BMP is the raw pixels, full stop

BMP is one of the oldest image formats still in circulation, and its defining feature is that it does not compress. Every pixel is written out in full — three bytes each, row after row. The arithmetic is blunt: a 1920 × 1080 screenshot is about two million pixels, times three bytes, roughly 6 MB — regardless of whether the image is a rich photo or a plain white dialog box.

That is also why BMPs still exist: writing one requires almost no code, so old Windows software, industrial tools, and quick-and-dirty exporters default to it.

The fix is one conversion

Re-encoding a BMP as JPG typically cuts it to a tenth of the size or less, because JPG actually compresses the picture. Our BMP to JPG converter does it in your browser — a folder at a time, nothing uploaded, with the size drop shown per file. For screenshots full of text where you want every edge pixel-crisp, convert to PNG instead: still far smaller than BMP, but lossless.

Does anything need to stay BMP?

Occasionally, yes: some legacy software and devices only read BMP. That is the one reason our converter also writes BMP — for feeding those systems. For every other purpose — storing, sending, uploading, publishing — BMP is pure overhead.

What we deliberately don't do

We don't shrink the pixel dimensions while converting. The dramatic size drop comes from encoding alone; your image keeps every pixel it had. If you want it smaller in dimensions too, that is a conscious step in resize, not a side effect.

The short version

• BMP stores raw uncompressed pixels — size depends only on dimensions, never content.
• Converting to JPG cuts it roughly tenfold; PNG for text-crisp screenshots.
• Keep BMP only for legacy software that reads nothing else.