BMP to PNG Converter

BMP files are almost always stored raw: three bytes per pixel, row after row, with no compression at all. The format allows run-length encoding for 4-bit and 8-bit images, but almost nothing writes it. PNG holds the same pixels and runs them through per-row prediction filters and DEFLATE, the compression method behind ZIP files, so repeated colors and smooth areas take a fraction of the space.

Bitmaps still turn up more often than you might expect. Older versions of Paint saved .bmp by default, the Windows clipboard keeps copied pictures as device-independent bitmaps, and scanners, machine-vision cameras, retro games and embedded devices write them because the format is trivial to produce. A folder of those files eats storage fast and is awkward to send.

The conversion does not sharpen or clean anything up. If the bitmap is blurry or blocky, the PNG is too; the gain is size and compatibility, not quality.

There is usually no transparency to gain. Most BMPs are 24-bit, which has no alpha at all, and the PNG comes out fully opaque. Some 32-bit bitmaps written with the version 4 or 5 header carry a real alpha channel, and that is preserved in the PNG. In many other 32-bit files the fourth byte is simply padding, and programs that misread it as alpha show the picture as invisible; if a converted file looks blank in one viewer, open it in another before assuming it is broken.

Low-color bitmaps, such as 1-bit scans and 8-bit palette images from old software, keep exactly the same colors. PNG supports grayscale, palette and full-color layouts, so it can represent every kind of BMP you are likely to meet. For a website, PNG is the right home for these screenshots and diagrams; our guide to reducing image size in KB covers what to do if a form still wants a smaller file.