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.
WebP is the smallest of the three for the web and can be lossless if you set quality to 100.
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 WebP 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.