BMP to PNG Converter

This tool converts BMP (bitmap) images to PNG in your browser. BMP is the oldest common image format and the least efficient one: it stores raw pixels, three bytes for every pixel, with no compression at all. That is why you rarely meet one on the web, and why they are so enormous when you do — a 12-megapixel photograph saved as a 24-bit BMP is about 35 MB on disk. PNG is the modern answer: it holds the same picture losslessly and typically takes a fraction of the space. The conversion is for anyone handed a .bmp by an old scanner, a Windows program, a laboratory instrument, an archived disc or a colleague, who needs it smaller, or in a format their website, editor or document will accept.

PNG is lossless, so nothing about the picture is thrown away. The browser decodes the BMP at its full stored size, draws the pixels onto a canvas of the same dimensions, and encodes that canvas as PNG — every pixel that came out of the BMP is in the PNG, exactly. There is no quality slider, because there is no quality to set: PNG's compression has no lossy setting to dial, so the only control would be a control that could not change anything. Transparency survives where the BMP had any — a 32-bit BMP can carry an alpha channel, and the PNG keeps it — so a cut-out stays cut out.

What does not survive is the file's metadata. A BMP carries no EXIF, no colour profile and no camera data, and none is added; colours are written as standard sRGB. A BMP cannot be animated either, so unlike a PNG-to-JPG conversion there is no first-frame warning here. Expect the PNG to be smaller, usually by a lot, but do not expect a fixed ratio: PNG's deflate compression does well on flat colour and poorly on photographic noise, so a photo-like bitmap still ends up fairly large.

Everything happens on your device. The BMPs are read into your browser's memory, converted there and handed back as downloads; nothing is transmitted, which you can confirm in the Network tab of your browser's developer tools — no request carries your file. Up to 50 files per batch, each up to 50 MB, downloadable individually or as a ZIP.

1. Each file's first bytes are checked for the BMP signature: the two ASCII characters "BM", followed by the file-size field, in a header of at least 26 bytes. A file that is really a PNG or JPG with a .bmp name is rejected with a message naming what it actually is, because the bytes decide, never the extension. 2. The browser decodes the BMP at its full stored size. Rows are stored bottom-up and padded to a multiple of four bytes; the decoder strips the padding and flips the rows, so the image comes out the right way up. 3. The decoded pixels are drawn onto a canvas of the same width and height — no resampling and no background painted, so any alpha the bitmap carried is kept. 4. The canvas is encoded with canvas.toBlob("image/png"). PNG's compression is lossless, so the output holds exactly the pixels the BMP decoded to. There is no quality parameter, because PNG has none. 5. The file keeps its original name with the extension changed to .png, and the size before and after is shown for every file.