BMP file format
BMP writes pixels to disk with almost no processing at all. That makes it enormous and effortless to read — the reason it still turns up inside Windows software.
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What is BMP?
BMP is the native bitmap format of Windows, dating from 1987. A file is a short header followed by the raw pixel values, row after row, with no compression in the usual case. A 1920×1080 photograph in 24-bit colour takes about 6 MB whether it is a detailed landscape or a solid white rectangle, because the format has no way to notice that the second one is repetitive.
Two quirks catch people out. Rows are stored bottom-up, so a naive reader that assumes top-down order produces a vertically flipped image. And each row is padded to a multiple of four bytes, so a 3-pixel-wide 24-bit image occupies 12 bytes per row rather than 9. Both are consequences of the format being designed around how Windows moved bitmaps into video memory.
That simplicity is exactly why BMP has not disappeared. Reading one requires a few dozen lines of code and no library, which suits embedded systems, test fixtures, screenshot pipelines and intermediate steps between tools. As a delivery format it makes no sense at all: PNG stores identical pixels losslessly at a fraction of the size.
| Extension | .bmp |
|---|---|
| Category | Image |
| MIME type | image/bmp |
| Released | 1987 |
| Developed by | Microsoft |
| Compression | None (uncompressed) |
| Transparency | Yes |
| Animation | No |
BMP: frequently asked questions
- Why are BMP files so large?
- Because they are normally uncompressed — every pixel is written out literally, so the size follows directly from width times height times bytes per pixel. Converting to PNG gives byte-identical pixels at typically a quarter to a tenth of the size, with no quality trade-off whatsoever.
- Is BMP lossless?
- Yes. Standard BMP stores raw pixel values with no approximation, so nothing is discarded. It is lossless in the same sense PNG is, only without the compression — which is why converting BMP to PNG is a pure win and converting to JPEG is not.
- Does BMP support transparency?
- A 32-bit BMP has a fourth byte per pixel that can act as an alpha channel, but support is inconsistent: many viewers ignore it and render the image opaque. If transparency matters, use PNG, where alpha is part of the specification and universally honoured.
- Why is my BMP upside down?
- Because BMP stores rows from the bottom of the image upward, unlike almost every other format. A reader that assumes top-down order produces a vertically mirrored picture. The header can also declare a negative height to signal top-down order, which some writers use and some readers mishandle.
- When should I still use BMP?
- When something must read the file with minimal code and no library — embedded devices, simple test harnesses, an intermediate step between two tools that both understand it. For storage, sharing or the web there is no case where BMP beats PNG or JPEG.
- Does converting BMP to PNG lose quality?
- No. PNG compression is lossless, so every pixel survives exactly and only the file size changes. You can convert back to BMP later and get an identical image, which makes PNG strictly the better way to keep the same data.