Windows bitmaps are usually uncompressed — megabytes of raw pixels. Drop them here and get JPGs a small fraction of the size, with the bit depth reported and the saving printed on every card.
What happens to a bitmap
The BMP's header is read first — width, height, bits per pixel, whether it is run-length encoded — and the row says what it found (“24-bit uncompressed BMP, 34.3 MB of raw pixels”). The browser then decodes the pixels and the JPG is written at the quality you choose; Measured gives every picture the lowest quality that keeps 40 dB of fidelity.
Screenshots and diagrams saved as BMP are the one case where a JPG is not automatically the best answer: sharp coloured text loses crispness at 4:2:0. Switch the encoder to Precise and choose 4:4:4, or let Auto pick it from the picture.
Step by step
- Drop one BMP or a folder of them.
- Keep Measured quality, or set a fixed number or a target size in KB.
- For screenshots or diagrams, choose the Precise encoder with Auto or 4:4:4.
- Press Download. Each card shows the bytes saved.
Old bitmaps, odd variants
Indexed 1-, 4- and 8-bit BMPs, 16-bit and 24-bit, and run-length encoded ones are read by the browser's own decoder. A file it cannot read fails on its own row with the reason, and the rest of the queue carries on.