Format
What Is BMP? The Bitmap Image Format Explained
BMP is Windows' native raster format from 1988 — uncompressed, very large, and now mostly superseded by PNG and JPEG for every practical use.
What is BMP?
BMP (Bitmap) is a raster image format developed by Microsoft for the Windows operating system. It was introduced with Windows 1.0 in 1985 and was the native image format built into the Windows GDI (Graphics Device Interface). Because Windows used it internally, BMP was the de facto format for Windows graphics through the 1990s.
BMP stores images as a direct grid of pixel values with minimal compression — effectively a raw dump of pixel data with a simple header. This makes it extremely simple to decode (no decompression step needed) but produces very large files.
How BMP works
A BMP file consists of:
- File header: 14 bytes identifying the file as BMP and recording the file size and pixel data offset.
- DIB header: Variable-length header (most commonly the BITMAPINFOHEADER at 40 bytes) describing image dimensions, color depth, and compression type.
- Color table: Optional palette for indexed color (1, 4, or 8 bits per pixel).
- Pixel data: Raw pixel values, stored bottom-to-top by default, padded to 4-byte row boundaries.
BMP optionally supports RLE (Run-Length Encoding) compression for 4-bit and 8-bit images — but in practice, most BMP files are uncompressed. There is no lossy compression option.
File size
BMP files are among the largest of any image format. Since most BMP files are uncompressed, file size scales directly with image dimensions and color depth:
- 1920×1080 @ 24-bit (RGB): ~6 MB
- 1920×1080 @ 32-bit (RGBA): ~8 MB
- 3840×2160 @ 24-bit (4K): ~24 MB
The equivalent JPEG (quality 85) is typically 300–600 KB. The equivalent PNG is typically 500 KB–3 MB depending on content. BMP's only advantage in size is for indexed-color graphics in 4-bit or 8-bit mode with RLE — but PNG still wins in those cases.
When to use BMP
In modern workflows, BMP has almost no use case where PNG isn't a better choice. The remaining situations where BMP is still encountered:
- Legacy Windows software: Older applications that only accept BMP for icons, splash screens, or custom graphics.
- Embedded systems and microcontrollers: Simple BMP parsing is easy to implement in low-resource environments; the lack of decompression overhead matters on underpowered hardware.
- Windows wallpapers and system graphics: Windows historically used BMP for desktop wallpapers, though modern Windows accepts most formats.
- Certain print workflows: Some industrial or scientific instruments output BMP as their native format.
For any general-purpose image storage, web delivery, photography, or design work — use PNG, JPEG, or WebP instead.
BMP vs alternatives
| Format | Compression | 1080p size | Browser | Best use |
|---|---|---|---|---|
| BMP | None (or RLE) | ~6 MB | Limited | Legacy Windows, embedded |
| PNG | Lossless DEFLATE | 0.5–3 MB | Yes | Lossless web/desktop |
| JPEG | Lossy DCT | 200–600 KB | Yes | Photos, web |
| TIFF (LZW) | Lossless LZW | 2–4 MB | No | Professional print/archive |
Frequently asked questions
Is BMP lossless?
Yes. Standard BMP stores raw, uncompressed pixel data — nothing is discarded. Each pixel is stored exactly as captured. However, lossless is table stakes; PNG is also lossless and produces files 10–20× smaller. The only advantage BMP has over PNG is simplicity of structure and zero decoding overhead, which matters in some embedded system contexts.
Why are BMP files so large?
Because BMP stores raw pixel data with no compression. A 1920×1080 BMP in 24-bit color stores 1920 × 1080 × 3 bytes = approximately 6 MB for every image regardless of content complexity. By contrast, JPEG can represent the same photograph in 200–400 KB and PNG in 500 KB–2 MB.
What opened BMP files before PNG existed?
BMP was the native image format for Windows from the earliest versions and was built directly into the Windows GDI (Graphics Device Interface). Every Windows application from the early 1990s could read BMP without any additional libraries, making it the default interchange format for Windows graphics before PNG and JPEG became widespread.