What Is BMP, and Why Does It Still Keep Turning Up?

The stubbornness of a forty-year-old format

When you come across a file with a .bmp extension on your computer, you almost certainly did not produce it. BMP is the raster image format Microsoft defined for Windows in the late 1980s, and there is virtually no reason left to choose it deliberately in a modern workflow. And yet it keeps turning up.

In this article we explain what the format actually is, why it takes up so much space, in which corners it survives today, and what you should do when a BMP lands in your hands.

How does BMP work?

To understand BMP it is enough to understand the format's philosophy: do the dumbest possible thing.

A BMP file consists of a short header block followed by a raw array of pixels. The pixel data is typically not subjected to any compression — the colour value of every pixel is written straight into the file. This is both the format's greatest weakness and its only real strength.

The weakness is obvious: the file size is entirely the product of resolution and bit depth. It has nothing to do with the image's content. A completely flat white square takes up almost the same space as a complex photograph of the same dimensions. The PNG version of the same image is far smaller despite being lossless, because PNG spots repeating patterns and compresses them.

The strength is this: you do not need a decoder to read a BMP. You can read a few numbers from the header and copy the pixel array straight into memory. Even a very constrained microcontroller can do that. That is largely why BMP has survived.

The specification defines a compression scheme called RLE (run-length encoding), but it is rarely used in practice and applies only to low bit depths. The overwhelming majority of BMP files you encounter are uncompressed.

The transparency question

BMP's 32-bit BGRA variant can carry an alpha channel per pixel, so transparency is theoretically possible. In practice it is a trap.

Alpha support is inconsistent across applications. Some programs interpret the alpha byte correctly; many ignore it entirely and display the image as opaque. That uncertainty means BMP cannot be trusted in any workflow where transparency matters. If you need transparency, the right format is PNG, and there is no debate about it.

Where do we encounter BMP today?

The format did not die because the systems that produce it did not die. The most common sources are these:

• Output from old Windows programs. Desktop software that has not been updated in years still offers BMP as an export option, sometimes as the only option.
• Old Paint saves. For a long time the default save format in Windows' classic Paint was BMP; archives from that era are full of them.
• Scanner and fax drivers. Some document scanning drivers deliver raw output as BMP, because they do not want to compress at driver level.
• Industrial and embedded devices. Production line cameras, measurement instruments and touch panel interfaces read and write BMP because they operate with limited processing power.
• Old game and CAD asset pipelines. Texture and sprite files were kept as BMP for many years; the source folders of those projects are still in circulation.
• Medical and laboratory equipment output. Some devices prefer to export measurement images in the simplest possible format.

So BMP is not a modern choice; it is a trace left behind by old systems.

Why do you need to convert it?

Using a BMP as it is almost always causes problems:

Size. Uncompressed files do not fit into email attachments, they needlessly fill cloud storage, and sharing them over mobile data is agony.

Platform rejection. Most social media, messaging apps and content management systems do not accept BMP uploads. When you try to upload one to a website, you will usually get a warning that the format is not supported.

Inconsistent browser behaviour. Some browsers display BMP, some do not. Relying on it for the web is illogical; it is indefensible on size grounds anyway.

Nothing in return. For putting up with all these problems you gain no quality advantage. PNG stores the same pixels losslessly and is far smaller besides.

What should you do with the BMP you have?

The decision tree is fairly simple:

If the content is a screenshot, text, drawing, logo or graphic: convert it to PNG. Because you are moving from lossless to lossless, not a single pixel changes, while the file shrinks dramatically. This is the safest default route for a BMP.

If the content is a photograph and the goal is to share it: converting to JPG gives the smallest file. It is a lossy operation and some data goes permanently, but on photographic content that trade is usually profitable.

If the file will be input to a program that expects BMP: leave it alone. This is the only justification for using BMP that still holds today.

In summary

BMP is a simple, old format that stores data in its rawest form. That simplicity makes it readable by constrained hardware but extremely inefficient for everyday use. If you want losslessness, PNG does the same job in far less space; if you want small files, JPG goes much further. The role BMP deserves today is not to produce anything from it, but to convert it to another format as soon as possible.