Why BMP files are so large
BMP, the bitmap format that has been part of Windows since its early versions, usually stores images without any compression: every pixel is written out, byte by byte. That makes BMP simple to read and very wasteful. A 1920 × 1080 image saved as a 24-bit BMP takes about 6 MB, because three bytes are stored for each of its roughly two million pixels.
BMPs still turn up regularly: exports from older Windows programs, scanners and scanning software, measuring and industrial equipment, legacy business applications and old games. They are awkward to email, often exceed upload limits, and many websites and forms do not accept them at all.
What JPG does to a bitmap
JPG compresses aggressively by discarding fine detail the eye is unlikely to miss. Applied to an uncompressed BMP, the reduction is usually dramatic, especially for photos and scanned pictures, and the JPG is often a small fraction of the BMP's size. At the default quality of 90 the difference from the BMP is usually hard to see in a photo.
The cost is that JPG is lossy, and the discarded detail cannot be recovered. That matters most for images with sharp edges: screenshots, diagrams, scanned text and pixel art can pick up soft edges and faint halos. For those, BMP to PNG gives a lossless file that is still usually much smaller than the BMP. Keep the original BMP if you may need the exact pixels again.
Choosing quality, size and background
- Quality: 90, the default, for photos and scans you want to keep; 80 to 85 for email and web forms. If the image contains text, such as a scanned letter, stay at 90 or above so the letters remain clean.
- Dimensions: scans in particular are often made at a resolution far higher than a screen needs. Settings → Size can scale them by a percentage or fit them within a maximum width and height, keeping the aspect ratio and never enlarging.
- Background colour: most BMP files have no transparency, so this setting rarely matters. If a BMP does contain transparent pixels, they are filled with this colour, white by default.
Treat the JPG as a finished copy. Editing and re-saving a JPG loses a little more quality each time, so make your edits on the BMP or a PNG version first, and create the JPG at the end.
BMP vs JPG
| BMP | JPG | |
|---|---|---|
| Compression | Usually none | Lossy |
| Size of a photo | Very large | Usually a small fraction of the BMP |
| Image data | Exact pixels | Some fine detail discarded |
| Transparency | Rare | Not supported |
| Web and email | Often rejected | Accepted almost everywhere |
| Best for | Legacy software and devices | Sharing, uploading, storing photos |
Notes on converting BMP files
- Batches: up to 10 BMPs at once, each up to 50 MB, converted one after another to keep memory use low.
- No upload: BMPs are decoded by your browser itself and never sent to a server, which matters for scans of private documents.
- ZIP download: the converted JPGs are stored in the ZIP as they are, without being compressed again.
- Metadata: EXIF metadata is not copied to the JPG.