JPEG vs Uncompressed BMP: File Size Comparison

When comparing digital images of identical pixel dimensions, a JPEG file is dramatically smaller than a raw uncompressed BMP file, typically reducing the required storage space by 85% to 95%. While an uncompressed BMP stores raw color data for every single pixel, resulting in a predictable and massive file size, JPEG employs lossy compression algorithms that strip redundant visual information. This difference in architecture means that an identical high-definition image can measure several megabytes in BMP format, yet only a few hundred kilobytes in JPEG format.

How BMP File Size Is Calculated

An uncompressed BMP (Bitmap) file has a deterministic file size directly tied to its resolution and color depth. Because it does not compress visual data, the file simply maps out every pixel across the width and height of the canvas.

For a standard 24-bit (True Color) BMP image, every pixel requires 3 bytes of data (1 byte each for red, green, and blue channels). The formula to calculate the data footprint is:

\[\text{File Size in Bytes} \approx (\text{Width} \times \text{Height} \times 3) + 54\text{ bytes (header overhead)}\]

Because content complexity does not affect uncompressed BMP files, a completely blank white canvas and a detailed photograph with the same dimensions will produce nearly the exact same file size.

How JPEG Achieves Smaller Sizes

Unlike BMP, JPEG is a lossy compression format. Its final size depends on both the user-selected quality level and the visual complexity of the image. JPEG reduces file size using several steps:

• Color Space Transformation and Chroma Subsampling: Converts RGB data to YCbCr (separating brightness from color) and discards high-frequency color data that the human eye cannot easily perceive.
• Discrete Cosine Transform (DCT): Converts spatial pixel blocks (usually 8x8 pixels) into frequency components.
• Quantization: Discards high-frequency details based on the selected quality setting. This is the primary stage where data is permanently reduced.
• Entropy Encoding: Compresses the remaining numerical data using algorithms similar to ZIP files (Huffman coding).

Because of this pipeline, flat or simple areas (like clear skies) compress far more efficiently than complex textures (like tree foliage).

Direct Size Comparison by Resolution

To see the stark contrast in file sizes, consider common display resolutions saved in both formats at standard 24-bit color depth:

• Full HD (1920 × 1080 pixels):
  • BMP: Approximately 6.22 MB
  • JPEG (High Quality, 85–90%): Approximately 300 KB to 700 KB
  • JPEG (Medium Quality, 60–70%): Approximately 150 KB to 250 KB
• 4K UHD (3840 × 2160 pixels):
  • BMP: Approximately 24.88 MB
  • JPEG (High Quality, 85–90%): Approximately 1.5 MB to 3.5 MB
  • JPEG (Medium Quality, 60–70%): Approximately 600 KB to 1.2 MB

Summary of the Difference

The typical compression ratio of JPEG compared to an uncompressed BMP ranges from 10:1 to 20:1 for standard photography, meaning the JPEG will be roughly 5% to 10% the size of the BMP. While BMP retains absolute, uncompressed pixel accuracy at the expense of storage, JPEG provides a vastly more efficient footprint with minimal visible degradation under normal viewing conditions.