BMP to JPG Converter

This tool converts BMP (bitmap) images to JPG (JPEG) files in your browser. BMPs are big because they are uncompressed: three bytes for every pixel, stored raw, with no compression at all. JPG is the most compact ordinary image format there is, so the same picture usually shrinks by an order of magnitude. You end up here holding an old scan, a screenshot captured by a desktop program, or a file from an instrument, and you need something a website, an email client or a document will accept without choking.

The trade-off is the one JPG always makes. JPG is lossy: it discards fine detail the eye is least likely to miss, and it does it by blending small blocks of colour together. That is what buys the small file. At the default quality of 92 the loss is very hard to see in a photograph, but sharp text and thin lines — the kind of content a desktop screenshot is full of — are where artefacts show first, as faint smudges around the edges. Lower the quality and the file gets smaller still, and so does the visible damage. Nothing about the picture improves in the process: the JPG holds exactly the pixels the BMP decoded to, minus the detail JPG chooses to discard.

Transparency cannot survive this conversion. JPG has no alpha channel at all, so any see-through pixels a 32-bit BMP might carry are replaced by the background colour you choose — white unless you change it. Almost every BMP in the wild is 24-bit and fully opaque, in which case there is nothing transparent and nothing is flattened; the control is there because the rare 32-bit file needs it, and the tool always says plainly which colour was used.

Everything happens on your device. The BMPs are read into your browser's memory, converted there and offered back as downloads; nothing is transmitted, which you can confirm in the Network tab of your browser's developer tools — no request carries your file. Up to 50 files per batch, each up to 50 MB, downloadable individually or as a ZIP.

1. Each file's first bytes are checked for the BMP signature — the ASCII characters "BM", the file-size field, and a header of at least 26 bytes. A file that is really a PNG or JPG with a .bmp name is rejected with a message naming what it actually is; the bytes decide, never the extension. 2. Quality (1–100) is validated and converted to the 0–1 scale the browser's encoder takes; 92 out of 100 is the default. The background colour is validated as a hex value and defaults to #ffffff. 3. The browser decodes the BMP at its full stored size, stripping the row padding and flipping the bottom-up rows the right way up. 4. A canvas of the same size is filled with the background colour and the bitmap is drawn over it, so any transparent pixel takes that colour. 5. The canvas is encoded with canvas.toBlob("image/jpeg", quality), and the file keeps its original name with the extension changed to .jpg. The size before and after is shown for every file.