What is the BMP file format?
.bmp is an image file that stores raster graphics, developed by Microsoft (with IBM, for OS/2). BMP format is almost always uncompressed, so high-resolution images saved as .bmp files are larger than files saved in comparable formats such as PNG or JPEG. Optional run-length encoding (RLE) compression exists but is rarely used.
BMP is capable of storing images in various color depths:
- 1-bit monochrome (black and white only),
- 4-bit (16 distinct colors),
- 8-bit (256 distinct colors),
- 16-bit, 24-bit, or 32-bit color (the latter adds an alpha channel for transparency).
File structure
Each .bmp file has a header section which contains information about the color depth, file size, and pixel data offset, followed by an array representing bitmap pixels. Rows are stored bottom-up and padded to 4-byte boundaries. Four DIB header versions exist - BITMAPCOREHEADER, BITMAPINFOHEADER, BITMAPV4HEADER, and BITMAPV5HEADER - the last of which adds embedded ICC color profiles.
Security & safety
RISK: LOWBMP is plain pixel data with no scripts or macros, so the format itself is low-risk. The only caveats: historically a few image-library bugs allowed malformed BMP headers (huge claimed dimensions, bad RLE runs) to crash or overflow parsers, so keep your viewer updated; and avoid ad-laden "free BMP converter" sites - use IrfanView, GIMP, ImageMagick or Squoosh instead.
Format details
in a nutshellFULL NAMEWindows Bitmap Imageaka Device Independent Bitmap (DIB), BMP file
DEVELOPERMicrosoft (with IBM, for OS/2)since 1985-1990 (Windows 1.0/2.0; OS/2 1.x); BITMAPINFOHEADER form with Windows 3.0 (1990)
CATEGORYRaster Image Files
MIME TYPEimage/bmp
TYPEUncompressed (or lightly RLE-compressed) raster image, binary
STANDARDOpen · royalty-free
MAGIC BYTES · FILE SIGNATURE
OFFSET
0001
HEX
424D
ASCII
BM
Standard files start with 'BM' (42 4D). Rare OS/2 variants use BA, CI, CP, IC or PT. Bytes 0x0A-0x0D give the pixel-data offset; the DWORD at 0x0E is the DIB header size, which identifies the header version (12=CORE, 40=INFO, 108=V4, 124=V5).
Programs that open BMP files
Windows5 apps
Photos (built-in) / Paint
Built-in Double-click to view in Photos, or open in Paint to edit and re-save. 
macOS2 apps
Linux2 apps
Web1 app
Technical details
deep spec| MIME type | image/bmp (also image/x-bmp, image/x-ms-bmp) |
| Magic bytes | 42 4D ("BM") at byte offset 0 |
| Byte order | Little-endian |
| Compression | Typically none (BI_RGB); optional BI_RLE8 and BI_RLE4 run-length encoding; rare BI_BITFIELDS, BI_JPEG, BI_PNG |
| Supported color depths | 1, 4, 8, 16, 24, or 32 bits per pixel |
| Pixel row storage | Bottom-up by default (last row first); negative height value signals top-down; each row padded to a 4-byte boundary |
| File header | BITMAPFILEHEADER - 14 bytes; contains BM signature, total file size, and pixel-data offset |
| DIB header versions | BITMAPCOREHEADER (12 bytes), BITMAPINFOHEADER (40 bytes), BITMAPV4HEADER (108 bytes), BITMAPV5HEADER (124 bytes); version identified by DWORD at offset 0x0E |
| Alpha channel | Supported in 32-bit mode via BITMAPV4HEADER and BITMAPV5HEADER; not available in 24-bit or earlier header versions |
| Maximum image dimensions | Up to ~2.1 billion pixels per side (32-bit signed int in BITMAPINFOHEADER); BITMAPCOREHEADER limits to 65,535 px per side |
| Color palette | Optional; up to 256 RGBQUAD entries used for indexed modes (1, 4, 8 bpp) |
| ICC color profile | Embedded ICC profile support added in BITMAPV5HEADER (Windows 98/2000); earlier headers carry no color-space metadata |
| Alternative extension | .dib (Device Independent Bitmap) - identical format, used interchangeably on Windows |
| OS/2 variants | OS/2 Bitmap Array files use alternative magic signatures: BA, CI, CP, IC, PT |
| Released | 1985-1990 (Windows 1.0/2.0; OS/2 1.x); BITMAPINFOHEADER form with Windows 3.0 (1990) |
| Latest version | BITMAPV5HEADER (Windows 98/2000, 1998) - adds ICC color profiles |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
BMP conversions
Possible
.BMP .JPGGUIDE
Yes, BMP converts to JPG without any special software - the fastest way on Windows is Paint (built-in): open…
How to convert
.BMP .PNGGUIDE
Yes, BMP converts to PNG without any loss - the fastest route is dragging the file into Squoosh in your…
How to convert
.BMP .GIFGUIDE
Yes, BMP converts to GIF without issue - IrfanView on Windows is the fastest route for a single file or a…
How to convert
.BMP .PDFGUIDE
Yes - on Windows, right-click the BMP and print to "Microsoft Print to PDF"; on macOS, open in Preview and…
How to convert
.BMP .TIFFGUIDE
Yes - open the BMP in IrfanView or GIMP and export as TIFF; the conversion is lossless and takes seconds.
How to convert
.BMP .PCXGUIDE
Yes - IrfanView or ImageMagick convert BMP to PCX in a few clicks, though PCX is rarely needed today.
How to convert
.BMP .ICOGUIDE
Yes, you can convert a BMP to ICO - the quickest method on Windows is IrfanView: open the BMP, File > Save…
How to convert
.BMP .PSDGUIDE
Yes, open the BMP in Adobe Photoshop or GIMP and save it as a PSD file.
How to convert
Community Q&A
asked by usersNo questions yet - be the first to ask about BMP files.
Frequently asked questions
How do I open a .bmp file?
Just double-click it - Windows Photos/Paint, macOS Preview, and tools like IrfanView, XnView, GIMP and Photoshop all open BMP natively. No special software is needed.
Why is my .bmp file so large?
BMP is normally uncompressed, storing every pixel directly, so files are big. Convert to PNG (lossless) or JPG (photos) to shrink them dramatically.
How do I convert BMP to PNG or JPG?
Open it in IrfanView, GIMP or Preview and 'Export/Save As' the new format, or use a local web tool like Squoosh. For many files, use IrfanView's or ImageMagick's batch mode.
Is BMP lossless?
Yes - standard uncompressed BMP keeps exact pixel values with no quality loss. That's why it's large; PNG gives the same losslessness with much smaller files.
Can a BMP have transparency?
Only 32-bit BMPs (with a V4/V5 header) carry an alpha channel, and support is inconsistent across apps. For reliable transparency, use PNG instead.
Can I convert a BMP to a vector (SVG/AI/DWG)?
Not directly - BMP is pixels, not shapes. You must trace it: Illustrator's Image Trace or Inkscape's Trace Bitmap turns the raster into editable vector paths.
References
1Microsoft Learn - Bitmap Storage (BMP file format)learn.microsoft.com
2Microsoft Learn - BITMAPINFOHEADER structurelearn.microsoft.com
