About

BMP (Bitmap) files store uncompressed raster data, resulting in file sizes 10× to 50× larger than equivalent PNGs. This converter performs lossless transformation from BMP to PNG format entirely within your browser - no server uploads, no file size limits, no privacy concerns. The PNG format uses DEFLATE compression achieving typical ratios of 2:1 to 10:1 while preserving every pixel exactly. Critical for web deployment where BMP files are unsupported by most browsers and CDNs.

The conversion preserves alpha channel transparency from 32-bit BMPs and handles legacy formats including 1-bit monochrome, 4-bit (16 colors), 8-bit (256 colors), 16-bit highcolor, and 24-bit truecolor. Processing occurs via the Canvas API using hardware-accelerated decoding. Batch conversion processes files sequentially to prevent memory exhaustion on large queues.

Formulas

The compression ratio achieved depends on image content entropy. For a typical photographic BMP:

R = S_BMPS_PNG

Where R is compression ratio, S_BMP is source file size in bytes, and S_PNG is output PNG size. Expected ratios: solid colors achieve R > 50, photographs typically 2 ≤ R ≤ 5, and noise-heavy images approach R ≈ 1.

Uncompressed BMP size calculation:

S_BMP = W × H × D8 + 54

Where W = width in pixels, H = height in pixels, D = bit depth, and 54 bytes is the standard BMP header size (BITMAPFILEHEADER + BITMAPINFOHEADER).

Reference Data

Property	BMP Format	PNG Format
File Extension	`.bmp`, `.dib`	`.png`
MIME Type	`image/bmp`, `image/x-ms-bmp`	`image/png`
Compression	None (RLE optional)	DEFLATE (lossless)
Transparency	32-bit only (alpha channel)	Full alpha channel
Color Depths	1, 4, 8, 16, 24, 32-bit	1, 2, 4, 8, 16-bit per channel
Max Dimensions	32,767 × 32,767 px	2³¹ − 1 px per side
Typical File Size (1920×1080 24-bit)	~6.2 MB	~1.5 - 3 MB
Web Browser Support	Partial (download only)	Universal
Animation Support	No	APNG extension
Metadata	Minimal (dimensions, color depth)	Extensive (tEXt, iTXt chunks)
Interlacing	No	Adam7 interlacing
Year Introduced	1986 (Windows 1.0)	1996 (W3C)
Gamma Correction	No	gAMA chunk supported
Color Profiles	Limited (v4/v5 headers)	iCCP chunk (ICC profiles)
Use Case	Raw capture, legacy Windows apps	Web, lossless editing, transparency

Frequently Asked Questions

No. PNG uses lossless DEFLATE compression, meaning every pixel value is preserved exactly. The conversion is mathematically reversible - converting PNG back to BMP produces bit-identical pixel data. Quality loss only occurs with lossy formats like JPEG.

Some legacy 32-bit BMPs use the alpha channel for padding rather than transparency. If your BMP was created by older software that stored garbage data in the alpha bytes, the converter will interpret this literally. Re-export from the source application with explicit alpha channel support enabled.

Browser memory limits apply - typically 500 MB to 2 GB depending on available RAM. For a 24-bit BMP, this translates to approximately 13,000 × 13,000 pixels maximum. Processing occurs entirely in-browser with no upload limits imposed by servers.

Yes. The browser's native image decoder handles RLE4 (4-bit) and RLE8 (8-bit) run-length encoded BMPs automatically. However, RLE24 compression (rarely used) may fail on some browsers - Firefox has the most complete BMP decoder.

Files are processed sequentially, not in parallel, to prevent memory exhaustion. Each conversion releases resources before the next begins. For batches exceeding 50 files or 200 MB total, expect processing times of 10-30 seconds depending on hardware. The UI remains responsive via asynchronous processing.