BMP to PNG Converter

BMP files keep every pixel uncompressed, which makes them big and awkward to email or upload. PNG keeps every pixel too, but compresses losslessly, so the same image is usually a fraction of the size with no change at all — ideal for screenshots, diagrams, icons and pixel art. This converter decodes each bitmap with your browser and saves a PNG, keeping the transparency of 32-bit BMPs that store an alpha channel. "Bitmap to PNG" means exactly this: BMP is the Windows bitmap format. Files never leave your device.

How to use this tool

1. Drop one or more .bmp files into the box.
2. Keep "Keep transparent" for 32-bit BMPs with an alpha channel, or fill those areas with white or black.
3. Optionally set a max width to scale large images down.
4. Download the PNGs; the table shows how much smaller each one is.

Formula

PNG = the same RGBA pixels, filtered row by row and compressed with Deflate (lossless). Size change = PNG bytes ÷ BMP bytes − 1.

Flat colours and repeated patterns compress very well, so screenshots often shrink by 90% or more; noisy photos shrink less. The bit depth, compression and header version of each BMP are shown in the table. Palette BMPs (1, 4 or 8 bits) are saved as full-colour PNG.

Example

An 800 × 600 px, 24-bit diagram.bmp is 54 + 600 × 2,400 = 1,440,054 bytes (each row is 800 × 3 = 2,400 bytes, already a multiple of 4). As PNG, a diagram with large flat areas typically needs well under 100 KB — over 90% smaller — and every pixel is identical.