About

Bitmap files (.bmp) store uncompressed raster data, resulting in file sizes of 10 - 50× larger than equivalent JPEGs. This converter applies DCT-based lossy compression to reduce file size while preserving acceptable visual fidelity. Quality settings map directly to JPEG quantization tables: Q = 100 retains maximum detail; Q = 50 typically achieves 90% size reduction with minimal perceptible degradation.

Processing occurs entirely in-browser via Canvas API. No server uploads, no privacy risks. The tool handles 24-bit and 32-bit BMP variants, including those with alpha channels (alpha data discarded during JPEG encoding, as JPEG does not support transparency).

Formulas

JPEG compression ratio depends on quality factor and image content entropy. The effective compression follows:

R = S_originalS_compressed

where R = compression ratio, S_original = BMP file size in bytes, S_compressed = JPEG output size.

Size reduction percentage:

Reduction = S_original − S_compressedS_original × 100%

BMP uncompressed size estimation for 24-bit images:

S_bmp ≈ W × H × 3 + 54

where W = width in pixels, H = height in pixels, 3 = bytes per pixel (RGB), 54 = BMP header size.

Reference Data

Quality Setting	Typical Compression Ratio	Visual Impact	Best Use Case
100%	5:1	Imperceptible loss	Archival, print masters
90%	10:1	Negligible artifacts	Photography, portfolios
80%	15:1	Minor artifacts in gradients	Web images, social media
70%	20:1	Visible on close inspection	Thumbnails, previews
60%	25:1	Noticeable blocking	Low-bandwidth delivery
50%	30:1	Obvious compression artifacts	Quick previews only
40%	40:1	Significant degradation	Extreme size constraints
30%	50:1	Heavy blocking, color banding	Placeholder images
20%	60:1	Severe artifacts	Testing, debugging
10%	80:1	Barely recognizable	Not recommended

Frequently Asked Questions

JPEG uses YCbCr color space internally, while BMP stores RGB directly. The conversion involves color space transformation that may cause subtle shifts, particularly in saturated reds and blues. Additionally, chroma subsampling (4:2:0 at lower quality) reduces color resolution.

JPEG does not support alpha channels. Any transparency data in 32-bit BMPs is discarded during conversion. Transparent regions typically render as white or black depending on how the canvas handles alpha compositing. For transparency preservation, convert to PNG instead.

This occurs with very small images or images with extremely low entropy (solid colors). JPEG overhead (headers, Huffman tables) can exceed savings for such cases. For images under 100×100 pixels, consider PNG format instead.

Quality 75-85 provides optimal balance for web delivery. This typically achieves 15:1 to 20:1 compression with artifacts invisible at normal viewing distances. Google PageSpeed recommends keeping JPEG quality below 85 to avoid diminishing returns.

No. BMP files do not contain EXIF data, and Canvas API-based conversion does not transfer metadata. If you need metadata preservation, use dedicated image editing software.

This tool processes one file at a time for precise quality control. For batch conversion, process each file sequentially. Each conversion completes in under 100ms for typical image sizes.