BMP does not compress at all
That is the entire story. A BMP is a header followed by every pixel written out one after another. The size depends only on the dimensions and colour depth, never on the content:
| Dimensions | BMP size (24-bit) | | --- | --- | | 800 × 600 | ~1.4 MB | | 1920 × 1080 | ~6 MB | | 3840 × 2160 | ~24 MB |
Anything else you convert to will be dramatically smaller, which is why this conversion has such a reliable payoff.
JPG gives the smallest result and is the right choice for a photograph or a scan. Transparency is not a consideration — BMP files almost never have any.
Where BMP still comes from
Windows Paint saves, older scanner software, screenshots taken by industrial or medical equipment, and exports from software written a long time ago. It is almost never a deliberate choice — it is what something old defaulted to.
Nothing leaves your machine
Given how large BMP files are, this matters practically as well as privately: a 24 MB file never gets uploaded anywhere, so the conversion is instant and works with the network off.
Questions
Why are BMP files so large?+
Because BMP stores every pixel literally, with no compression at all. A 1920×1080 image is about 6 MB regardless of what it shows — a blank white rectangle costs exactly as much as a photograph. Every other format on this page compresses.
How much smaller will it be?+
Usually 90% or more. The exact figure appears in the results table. A photograph converted to JPG routinely goes from megabytes to tens of kilobytes.
Is anything lost?+
For PNG and WebP at quality 100, nothing — the pixels are identical. For JPG, a little detail is discarded, which is invisible on a photograph and can be noticeable on text or a hard-edged diagram.