About

BMP (Bitmap) files store uncompressed pixel data, resulting in file sizes often 10× to 50× larger than equivalent JPEGs. A 1920×1080 24-bit BMP weighs approximately 5.93 MB, while JPEG compression at 85% quality reduces this to under 500 KB - a 90% reduction. This converter performs native browser-based decoding of BMP binary structures including 24-bit RGB and 32-bit RGBA formats, handling bottom-up scanline order and 4-byte row padding per the Microsoft BMP specification. Quality degradation follows the DCT (Discrete Cosine Transform) compression model where values below 70% introduce visible artifacts in high-frequency regions.

Critical for web deployment: BMP format lacks browser support in img srcset and most CDN optimization pipelines reject it outright. Batch conversion eliminates manual processing bottlenecks when migrating legacy image assets. The tool validates BMP magic bytes (0x42 0x4D) to reject corrupted or misnamed files before processing.

Formulas

BMP file size calculation follows a deterministic formula based on image dimensions and color depth. Row padding ensures each scanline aligns to a 4-byte boundary for memory efficiency on 32-bit systems.

S = 54 + P + ceil(W × D32) × 4 × H

Where S = file size in bytes, W = width in pixels, H = height in pixels, D = bit depth, and P = palette size (0 for 24/32-bit, 2^D × 4 for indexed modes).

RowBytes = ceil(W × D8)

Padding = (4 − RowBytes mod 4) mod 4

JPEG compression ratio estimation uses empirical coefficients based on quality factor Q (0 - 100):

R ≈ 100Q + 2

Where R = compression ratio. At Q = 85, expect approximately 1:3.2 compression versus uncompressed RGB data.

Reference Data

BMP Type	Bit Depth	Colors	File Size (1920×1080)	JPG Equivalent (85%)	Compression Ratio
Monochrome	1-bit	2	253 KB	45 KB	5.6:1
16-Color	4-bit	16	1.01 MB	120 KB	8.4:1
256-Color	8-bit	256	2.03 MB	180 KB	11.3:1
High Color	16-bit	65,536	4.06 MB	320 KB	12.7:1
True Color	24-bit	16.7M	5.93 MB	450 KB	13.2:1
True Color + Alpha	32-bit	16.7M + Alpha	7.91 MB	480 KB	16.5:1
JPEG Quality Settings Reference
Maximum	100%		Minimal compression		2:1 to 4:1
High	85% - 95%		Imperceptible loss		8:1 to 15:1
Medium	70% - 84%		Slight softening		15:1 to 25:1
Low	50% - 69%		Visible artifacts		25:1 to 50:1
Minimum	10% - 49%		Heavy blocking		50:1 to 100:1
BMP Header Structure
File Header	14 bytes		Signature (BM), file size, pixel data offset
DIB Header	40 bytes		Width, height, bit depth, compression type
Color Table	Variable		4 bytes per color (indexed modes only)
Pixel Array	Variable		Row-padded to 4-byte boundaries
Common BMP Sources
Windows Screenshots	Legacy systems		Pre-Windows 8 default format
Scanner Output	Raw scans		Uncompressed archival preservation
Medical Imaging	DICOM export		Converted from proprietary formats
CAD Software	AutoCAD, SolidWorks		Raster export from vector
Game Assets	Texture sources		Uncompressed development files

Frequently Asked Questions

JPEG format does not support alpha transparency. The converter composites transparent pixels against white during the conversion process. If preserving transparency is required, export to PNG format instead. The alpha channel data is correctly parsed from the BMP but necessarily discarded during JPEG encoding.

8-bit BMPs use indexed color with a maximum palette of 256 colors. The conversion first expands this to full RGB before JPEG compression. If the original palette contained dithering patterns to simulate more colors, JPEG's DCT compression may amplify these artifacts. Use quality settings above 90% for indexed source files.

BMP format requires each scanline to align to 4-byte boundaries. A 24-bit image 1 pixel wide wastes 1 byte per row (3 bytes data + 1 padding = 4). A 1001-pixel wide 24-bit row uses 3003 bytes, padded to 3004. This overhead becomes negligible at typical image widths but can inflate small or narrow images by 1-3%.

Web deployment typically targets 75-85% quality, balancing visual fidelity against bandwidth. At 85%, most images show no perceptible degradation on standard displays. Archival conversion should use 92-95% to preserve detail for potential future re-encoding. Never use 100% as it provides minimal quality improvement at 3-4× file size cost.

The tool supports Windows BITMAPINFOHEADER (40-byte DIB header) which covers 99% of BMP files created after 1995. OS/2 BITMAPCOREHEADER (12-byte) and other variants including BITMAPV4HEADER and BITMAPV5HEADER with ICC color profiles will be rejected with a specific error message indicating unsupported header type.

JPEG compression efficiency depends on image content. High-frequency detail (text, fine lines, noise) resists DCT compression regardless of quality setting. A BMP screenshot of code may compress only 5:1 at 50% quality while a photograph achieves 30:1. Additionally, JPEG adds metadata overhead of approximately 500-2000 bytes per file.