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BMP
What is a BMP file?
Uncompressed Windows bitmap. Huge files, still produced by scanners and older software.
What BMP is
BMP is a binary format, meaningful only to a program that knows it. It stores a grid of pixels, so enlarging it past its own resolution softens it. It is used for moving data between programs.
The extension is .bmp, and the full name is Windows Bitmap. Both matter less than what the file can hold, which is what the rest of this page is about.
Where BMP came from
Microsoft published it in 1990.
A format that has been readable for that long is a format worth trusting with something you want back in ten years.
The specification is published, but by its owner
It is written down and implementable, though the vendor decides what the next version looks like. In practice that is enough for the file to keep opening; it is not enough to be sure of it.
It is a legacy format
It still opens everywhere and is still written by older tools, but nothing new is being built around it. Convert what you mean to keep, and do not choose it for something starting today.
Nothing is thrown away
BMP stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.
It can be transparent
BMP carries an alpha channel, so a logo keeps its soft edges over whatever sits behind it instead of arriving in a white box.
Colour it can describe
Up to 8 bits per channel. It can work in RGB and an indexed palette.
Eight bits per channel is what a screen shows and what nearly every delivery format carries.
What opens BMP
Microsoft Paint, GIMP and IrfanView read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
Every current browser reads it.
That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.
It is a delivery format
BMP is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
What goes wrong with it
The recurring complaints: the files are large for what they contain.
None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.
The facts, in one place
| Extension | .bmp, .dib |
|---|---|
| Media type | image/bmp |
| Published by | Microsoft |
| First published | 1990 |