BMP を JPG に変換

A 24-bit BMP spends three bytes on every pixel, with no compression at all. A 1920×1080 image is 1920 × 1080 × 3 = 6,220,800 bytes of pixel data plus a 54-byte header, roughly 5.9 MB, whatever the picture shows. A 4000×3000 scan is about 36 MB. BMP still turns up from older versions of Windows Paint, scanner software, industrial cameras, lab equipment and legacy screenshot tools, and many email services and upload forms either reject it or balk at the size.

JPG compresses the same photo or scan by a large factor: a 6 MB BMP photo commonly lands under 1 MB at 85–90% quality. The exact ratio depends on how much fine detail and noise the image contains.

The slider runs from 10% to 100% and starts at 90%. For photos and scanned documents, 85–90% looks the same as the BMP at normal viewing size. For screenshots with small text, JPG can leave faint ringing around letter edges; keep the quality at 90% or higher, or convert the screenshot with Image to PNG, which stays lossless and is often smaller than a JPG for flat interface graphics. Settings around 70% suit images that only need to be recognisable, such as a photo attached to a support ticket.

The browser decodes the BMP, draws it onto a canvas and encodes a JPG at the chosen quality. Dimensions stay the same: a 1024×768 BMP becomes a 1024×768 JPG. A 32-bit BMP with an alpha channel loses its transparency, since JPG has none, and transparent areas are filled with white. Palette BMPs with 256 colours or fewer are expanded to full colour, which is harmless. BMP files carry no EXIF block, so no metadata is lost. If one file in a batch uses a variant your browser cannot decode, that file shows an error and the rest still convert.

Scans and equipment exports are often confidential, such as signed contracts or lab images, so the conversion runs entirely in your browser. The BMP is read from your disk, converted in memory and returned as a JPG download; nothing is uploaded or stored. Add up to 30 files per drop at 50 MB each, enough for a 4000×4000 BMP, and download the JPGs one at a time or as a single ZIP.