BMP Format Specification: Uncompressed Raster Graphics Architecture
Understand the BITMAPFILEHEADER, bottom-up pixel array storage, bit depths, and client-side WebP compression.
Format Specification & Header Signature
Core file signatures, developer metadata, and standard RFC specifications:
| Magic Bytes (Hex Signature) | 42 4D (BM - Bitmap header) |
| MIME Media Type | image/bmp, image/x-ms-bmp |
| Standard / Specification | Device-Independent Bitmap (DIB) Specification |
| Initial Developer / Organization | Microsoft Corporation & IBM (1985 (Windows 1.0)) |
| File Signature | 2 bytes: 0x42 0x4D ("BM") |
| Header Size | 14-byte BITMAPFILEHEADER + 40-byte BITMAPINFOHEADER |
| Pixel Storage Order | Bottom-up scanlines (row 0 is bottom of image) |
| Compression Support | Uncompressed RGB (standard) or Run-Length Encoding (RLE) |
Operating System & Browser Compatibility Matrix
Where .bmp, .dib is natively supported without third-party codecs:
| Platform / Environment | Native Support | Implementation Notes |
|---|---|---|
| Universal System Support | Supported | Native rendering in 100% of Windows versions, macOS, Linux, and image editing suites. |
Advantages of Bitmap Image File
- •True raw quality: zero lossy compression artifacts or color subsampling degradation
- •Trivial to parse programmatically: direct memory copy of pixel bytes into graphics framebuffers
- •Historical standard for Windows graphical user interface icons and wallpaper bitmaps
Limitations & Trade-Offs
- •Extreme file sizes: an uncompressed 4K BMP image exceeds 25 Megabytes
- •Completely impractical for web delivery due to bandwidth consumption and slow load times
- •Lacks modern alpha transparency and color management profiles found in PNG and WebP
The Bottom-Up Row Storage Architecture
A unique quirk of the BMP format is that image rows are typically stored "bottom-up": the first pixel in the byte array corresponds to the bottom-left corner of the image, progressing upwards.
Each scanline is padded with null bytes so that every row ends on a 4-byte (32-bit DWORD) memory boundary, facilitating fast memory transfer into early x86 VGA hardware.
Client-Side Tools for Bitmap Image File (.bmp, .dib)
Convert, optimize, or inspect .bmp, .dib files directly inside your browser with zero server uploads:
Image Compressor (JPG, PNG & WebP)
Reduce photo file size for web publishing, email attachments, and online forms with instant live preview.
Image Resizer (Passport & Custom Dimensions)
Quickly resize photos for passport applications, visa forms, resume uploads, or social media with aspect ratio lock.
HEIC to JPG / PNG Converter (iPhone Photo Fix)
Fix unopenable Apple .HEIC and .HEIF images on Windows, Linux, and Android. Fast, private WebAssembly conversion in your browser.
Frequently Asked Questions about Bitmap Image File
Why should I convert BMP images to WebP or JPG?
Converting uncompressed BMP images to modern WebP shrinks file size by over 95% with zero perceptible loss in visual quality, drastically speeding up websites and email attachments.