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File Format Guide
What is a BMP file?
BMP is about as simple as an image format gets: a small header followed by the colour of every pixel, one after another. That simplicity made it the Windows standard for years, and it is also why a BMP is enormous.
BMP at a glance
What is a BMP file?
An uncompressed raster image format. It stores color data for every individual pixel, resulting in extremely high quality but massive file sizes.
How to open a BMP file?
Microsoft Paint, Windows Photos, Apple Preview, and almost any image viewer/editor.
Typical use of a BMP file:
Historically used as the standard Windows graphic format. Today, it is rarely used outside of legacy software or specific programming environments.
Origin of the BMP file:
Developed by Microsoft in the 1980s for the Windows operating system.
Modern alternatives for BMP:
PNG provides the exact same lossless quality but with efficient compression, making files a fraction of the size.
BMP specifications
Full name
Bitmap / Device Independent Bitmap
File extensions
.bmp, .dib
MIME type
image/bmp
Compression
Usually none; optional RLE for 4- and 8-bit images
Transparency
Only in 32-bit variants, and inconsistently supported
Animation
Not supported
Colour depth
1, 4, 8, 16, 24, or 32 bits per pixel
Introduced
1986, with early Windows and OS/2
Browser support
Supported by all major browsers
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No compression, no surprises
A typical BMP stores three bytes per pixel with no compression at all. A 4000 × 3000 photograph is therefore about 36 MB, every time, regardless of content — a blank white image costs exactly as much as a detailed one. The same photograph as a JPEG might be 2 MB and as a PNG perhaps 20 MB.
There is a genuine benefit hiding in that inefficiency: reading a pixel requires no decoding, just arithmetic to find its offset in the file. On 1980s hardware, where a decompression step was expensive, that mattered enormously. On modern hardware it does not, and the format has been comprehensively outclassed.
Quirks that trip people up
BMP stores rows bottom-to-top by default, a legacy of how early display hardware addressed memory. Colour values are stored blue-green-red rather than red-green-blue. Each row is padded to a multiple of four bytes. None of this matters when you are simply viewing a file, but it accounts for a good share of the bugs in hand-written BMP loaders.
Transparency is the other trap. The 32-bit BMP variant has a fourth byte per pixel that can hold alpha, but the original specification never defined what it meant, so support is inconsistent — some software honours it, some ignores it, and some treats the image as opaque. If you need reliable transparency, BMP is the wrong format.
Where BMP still turns up
You will still meet BMP in Windows system resources and icon internals, in output from older scanners and industrial or medical imaging equipment, in embedded systems and microcontroller displays that cannot afford a decoder, and in screenshots from legacy software. Some scientific instruments write BMP specifically because it is trivially parseable.
For essentially any other purpose, PNG is a strict improvement — identical lossless quality, proper transparency, and typically a fifth of the size.
Convert BMP files in your browser
Zolvo's image converter runs entirely on your own machine — your files are never uploaded to a
server. Drop a BMP file in and pick an output format, or jump straight to a common one:
Because they are stored uncompressed — the file records the colour of every pixel individually with no attempt at compression. Converting to PNG gives a pixel-identical result at roughly a fifth of the size, or less.
Is BMP better quality than PNG?
No. Both are lossless, so a BMP and a PNG of the same image contain exactly the same pixels. PNG simply stores them more efficiently, and adds proper transparency support that BMP lacks.
Does BMP support transparency?
Only in the 32-bit variant, and unreliably — the alpha channel was never properly specified, so software disagrees on whether to honour it. Use PNG if transparency matters.
What should I convert a BMP to?
PNG if you want to keep it lossless, which is almost always the right answer. JPG only if the image is a photograph and file size matters more than perfect fidelity.
