BMP files: raw pixels, and why they are so big
What a BMP actually stores, why it is many times larger than a PNG of the same image, and when anything still needs it. Convert BMP in your browser, no upload.
At a glance
- Type
- Image
- Compression
- Lossless
- Extensions
- .bmp
- MIME type
- image/bmp
- Also known as
- Bitmap
A BMP is about as simple as an image file can be: a short header followed by the pixels, written out as raw bytes in rows. Microsoft introduced it with Windows in 1985 and its appeal was that reading and writing it required almost no code. There is a compression option in the specification, but it is rarely used, so in practice a BMP is uncompressed.
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That is why the files are enormous. A modest screenshot can be tens of megabytes, and a photograph from a modern camera would be over a hundred. Converting to PNG stores exactly the same pixels with real lossless compression and typically cuts 60% to over 90% of the size — one of the few conversions that costs nothing at all and gains a great deal.
BMP survives because simplicity is occasionally worth more than size. Windows tools written against the native bitmap APIs, scientific and industrial instruments, microcontroller display code and some firmware pipelines read raw pixel data directly and have no image decoder at all. For those, BMP is a requirement rather than a preference. Note that the BMP this site writes has no alpha channel, so converting a transparent PNG to BMP fills the transparency with white.
Convert BMP to other formats
Convert other formats to BMP
FAQ
Why is my BMP so large?
Because it stores pixels as raw bytes with essentially no compression. The specification allows compression but almost nothing uses it.
Is converting BMP to PNG lossless?
Yes, entirely. Both store exact pixels, so the image is identical and only the file size changes — usually by 60% to over 90%.
BMP to PNG or to JPG?
PNG for screenshots, diagrams and text, where it stays exact. JPG for photographs, where it is far smaller and the loss is invisible.
Why does BMP still exist?
Because it is trivial to read and write, which suits older Windows software, scientific instruments, microcontroller code and firmware pipelines that have no image decoder.
What happens to transparency converting to BMP?
It is filled with white. The BMP written here has no alpha channel, so a transparent background becomes opaque.
Is there any reason to keep a BMP?
Only if a specific tool demands it. For every other purpose PNG holds the identical image in a fraction of the space.