Two lossless formats, one sensible direction
BMP and PNG both store pixels without throwing anything away, so going from one to the other changes nothing about the picture. What changes is the container. A BMP is essentially a raw dump of pixel rows with a small header, and unless a rare RLE variant is used, it is completely uncompressed. PNG wraps the same pixels in a filter step followed by deflate compression, the algorithm behind zip files. That is why a Paint drawing that is 3 MB as BMP can be under 100 KB as PNG with no loss whatsoever.
BMPs still turn up from scanner bundles, Windows screenshot utilities, CAD and GIS exports, game texture tools and scientific cameras. Almost nothing on the web accepts them, whereas PNG opens everywhere, which makes this the default conversion when you want the same image in a form that other people can use.
What the converter does with the pixels
The browser decodes the BMP into a bitmap, a Web Worker draws it onto an offscreen canvas, reads the raw RGBA pixels back and hands them to the WebAssembly PNG encoder from jSquash. There is no quality slider because PNG has no lossy mode; the encoder simply compresses at a fixed level. The output is always a 32-bit RGBA PNG, even when the source was a 24-bit or an 8-bit indexed BMP. That is still lossless, but it means a palette image is stored with more bits per pixel than strictly necessary. A dedicated optimiser could squeeze such a file further; this tool does not attempt that.
Transparency deserves a word. Most BMPs have none. Some 32-bit files written by modern tools carry an alpha channel, and browsers do not treat that channel consistently: some honour it and some read the image as fully opaque. If a transparent BMP comes out with a solid background, that is the decoder's interpretation rather than something the encoder discarded.
How much smaller it gets
PNG compression depends heavily on content. Flat colours, text, line art and UI screenshots compress dramatically; photographs and noisy scans barely compress at all.
- A 1366x768 desktop screenshot of 3.1 MB typically becomes 80 to 250 KB.
- A 2000x1500 line drawing or chart of 9 MB usually lands under 300 KB.
- A 12 megapixel photograph of 36 MB tends to end up at 15 to 22 MB, which is still huge.
Those are typical figures, not promises. The last case is the important one: for photographs, lossless PNG is the wrong destination. Use BMP to JPG or BMP to WebP, where a quality of 80 to 85 gets the same photo down to a few hundred kilobytes with no visible difference.
Memory and large files
Because the entire image is held as RGBA in the worker, four bytes per pixel, a 36 MB BMP needs roughly 48 MB for the decoded pixels alone, plus the source, plus the PNG being built. Desktop browsers cope with that comfortably. Phones with limited memory can fail on very large files, and PNG encoding of a 12 megapixel image takes a noticeable moment even on a fast machine. Nothing is sent anywhere during that time; the delay is your own CPU doing the compression.
When this conversion is worth doing
Convert when you need to upload, attach or embed the image, when you are archiving screenshots or drawings and want them small but exact, or when a tool refuses BMP. Skip it if the file will only ever live in software that reads BMP natively, since you gain nothing but a step. If you are unsure whether the content wants a lossless or a lossy format, PNG vs JPG walks through the decision with examples.