From the heaviest common format to one of the lightest
A BMP is about as uncompressed as an image file can be: three bytes per pixel, row after row, with a 54-byte header on the front. WebP sits near the other end of the scale. It was built by Google from the VP8 video codec to make web images smaller, and in lossy mode it routinely delivers files a twentieth or a fortieth of the size of the same picture as BMP. Converting between the two is the single biggest size reduction on this site that still keeps broad software support.
Typical BMP sources are Windows Paint and screenshot tools, scanner software, capture cards, game engines and lab equipment. Typical WebP destinations are websites, WordPress media libraries, documentation, chat and any place where bandwidth matters.
What the encoder does
The browser decodes the BMP, and a Web Worker on your device runs the libwebp encoder compiled to WebAssembly. The output on this page is always lossy WebP, controlled by the quality slider from 20 to 95. There is no lossless WebP option here; if you need exact pixels, BMP to PNG is the lossless route.
WebP compresses in blocks with prediction from neighbouring pixels, so smooth areas and gradients cost almost nothing, while noise, film grain and fine texture cost the most. Compared with JPG at the same file size, WebP holds edges more cleanly and shows fewer blocky artefacts in dark areas, which is why screenshots survive WebP better than they survive JPG. Transparency is supported: if a 32-bit BMP contains an alpha channel and the browser honours it, the WebP keeps it. BMP has essentially no metadata, so nothing is lost there.
Quality settings that make sense
For photographs and colour scans, 75 to 85 is the sweet spot; the default of 80 sits in the middle of it. For UI screenshots with text, use 85 to 92 so small letters stay sharp. Only go under 60 when you are making thumbnails and the source will be viewed at a fraction of its size.
A worked example: a 1920x1080 game screenshot saved as a 6.2 MB BMP typically becomes 150 to 300 KB at quality 80 and 350 to 500 KB at quality 90. A 2480x3508 scanned page of 26 MB usually lands between 700 KB and 1.5 MB at quality 85. A noisy 12 megapixel camera frame of 36 MB might still be 2 to 4 MB at quality 85, since grain does not compress. None of these are guarantees; the content decides.
Where WebP will and will not open
Every current browser displays WebP, as do Windows 10 and 11 Photos, macOS Preview since Big Sur, GIMP, recent Photoshop and nearly all web platforms. The holdouts are older desktop software, some email clients rendering inline images, and a few print and pre-press workflows. If the file is bound for one of those, BMP to JPG is the conservative choice, and BMP to AVIF is the more aggressive one for sites where you control the audience.
Batch use and memory
You can drop many BMPs at once. They are converted one after another in the same worker, and each WebP downloads separately with the original name and a new extension. The full image is held in memory as RGBA during the process, so a 36 MB BMP needs more than 100 MB of working space between the source, the decoded pixels and the output. Laptops and desktops handle that easily; a budget phone might not. Keep the BMP when it is a master you will keep editing, since every lossy save loses a little. For the broader comparison of modern formats, see WebP vs AVIF vs JPEG.