How the format lays out pixels
A BMP file begins with a small file header, follows it with an information header describing width, height and bit depth, and then writes the pixel array itself. Rows are stored bottom-to-top by convention and each row is padded out to a four-byte boundary, a quirk that catches out many first-time parsers. Colour can be stored at 1, 4, 8, 16, 24 or 32 bits per pixel, with the lower depths using a palette table that sits between the header and the pixels. Because the data is rarely compressed, what you see on disk is essentially a memory dump of the image.
The format is proprietary to Microsoft but thoroughly documented, which is why almost every imaging library can read and write it without licensing concerns. Several header revisions exist, and the later ones add fields for colour-space and gamma information that most software quietly ignores.
Why it has aged the way it has
BMP was designed for an era when displaying an image quickly mattered more than storing it efficiently. On modern systems its lack of compression makes files enormous compared with PNG or JPEG, so it has retreated to niches: clipboard transfers, simple icons, test fixtures and the occasional Windows system asset. It remains useful precisely because it is dumb — there is no entropy coder to debug and no colour transform to second-guess.
Strengths
- Trivially simple to read and write, with no decompression step.
- Stores exact, lossless pixel values with no generation loss.
- Universally supported across Windows software and imaging libraries.
Limitations
- Files are very large because the data is usually uncompressed.
- The classic 24-bit form carries no alpha (transparency) channel.
- Bottom-up row order and row padding trip up naïve decoders.
When to use BMP
Reach for BMP when you need a guaranteed-lossless, dependency-free image that any Windows tool can open, and file size is not a concern — otherwise convert it to PNG.
BMP FAQ
- Does BMP support transparency?
- The common 24-bit BMP has no alpha channel, so it cannot store transparency. A 32-bit variant does carry an alpha byte per pixel, but support for it is inconsistent across software, so PNG is the safer choice when you need clean transparency.
- Why are BMP files so much bigger than PNG?
- BMP normally stores every pixel uncompressed, so a photo takes roughly width × height × bytes-per-pixel on disk. PNG applies lossless compression and typically shrinks the same image several times over with no quality loss.
- Can I convert a BMP to PNG or JPEG in the browser?
- Yes. Morphr decodes the BMP and re-encodes it to PNG, JPEG or WebP entirely on your device, so the file never leaves your computer. PNG keeps it pixel-perfect, while JPEG gives a much smaller file for photographs.