Formats/BMP
What Is a BMP File? The Uncompressed Bitmap Format Explained
BMP (short for bitmap, with the .bmp extension) is one of the oldest image formats still in circulation, dating to the earliest versions of Microsoft Windows. It is about as simple as an image file gets: it stores the colour of every single pixel, in order, with essentially no compression. That simplicity made it easy for early software to read and write, but by modern standards it is wasteful, and in almost every situation another format will serve you better.
How BMP stores an image
A BMP file is a straightforward grid of pixel values with a small header describing the width, height and colour depth. Because it typically applies no compression, the file size is roughly predictable from the dimensions alone: every pixel takes up its full share of space whether the image is a complex photo or a plain white square. There is no clever encoding, no discarding of redundant data — just raw pixels written out one after another.
This makes BMP technically lossless (nothing is thrown away) but enormously inefficient. A modest image that would be a few hundred kilobytes as a PNG or JPG can be several megabytes as a BMP, because PNG achieves the same lossless result with real compression and JPG achieves a far smaller file by discarding invisible detail.
Why you rarely want a BMP today
The raw, uncompressed nature that once made BMP convenient is now its biggest drawback. The files are far larger than they need to be, they mostly lack transparency support, and web browsers generally will not display them, so a BMP is a poor choice for the web, for email, or for storage. Anything BMP can do, PNG does losslessly at a fraction of the size, and JPG does even smaller for photographs.
You still occasionally run into BMP as the output of older software, some Windows utilities, or certain scanners and industrial devices that never modernised. In those cases the file is perfectly valid — it is just unnecessarily large, and the sensible first step is almost always to convert it.
When BMP is (occasionally) the right call
There are narrow cases where BMP's simplicity is a genuine advantage: some legacy or embedded software can only read BMP, and because the format is trivial to parse, a few specialised tools and hardware devices prefer it. If a program specifically asks for a BMP, give it one — that is exactly the situation the format still exists for.
For everything else — sharing, publishing, storing, emailing — there is no reason to keep an image as BMP. Convert it to PNG if you need to preserve every pixel exactly (a screenshot, a logo, line art) or to JPG if it is a photograph and you want it small. Either will be smaller and far more widely supported.
How to open and convert a BMP
The built-in image viewers on Windows and macOS open BMP files without any extra software, though most web browsers will not display them. To make a BMP practical, convert it: choose PNG to keep it lossless and gain transparency and universal support, or JPG to shrink a photographic image dramatically. An image converter does this in seconds, and OfficePad's runs entirely in your browser, so even a large uncompressed bitmap is never uploaded anywhere.
Frequently asked questions
Why is a BMP file so large?
BMP stores every pixel raw with essentially no compression, so its size depends only on the image dimensions. PNG achieves the same lossless quality with real compression, at a fraction of the size.
How do I open a BMP file?
The built-in photo viewers on Windows and macOS open BMP files directly. Web browsers usually will not display them, so convert to PNG or JPG to view and share anywhere.
Should I convert BMP to PNG or JPG?
Convert to PNG for screenshots, logos and line art where you want every pixel preserved, or to JPG for photographs where you want the smallest possible file.
Work with BMP on OfficePad
See also in the glossary: Raster image, Lossless compression, Bit depth, RGB.