How to calculate the size of an uncompressed, and a TIFF Group 4 compressed, bitonal scanned image
All scanned images consist of rows of pixels. In bitonal (black and white) images, each of these pixels is represented by 1 bit of data.
The image resolution is measured in pixels (or dots) per inch. The most technically correct way to display the resolution is to indicate both the horizontal resolution and vertical resolution.
However, when the horizontal resolution matches the vertical resolution, it is common to express the resolution using a single number that applies to both the horizontal and vertical resolutions:
The formula for calculating the size of a uncompressed image is:
Width (in inches) x horizontal DPI x Height (in inches) x vertical DPI x bit depth / 8 (bits in a Byte) / 1024 (Bytes in a KB)
When scanning a letter sized (8.5x11 inch) page at 200dpi, black and white:
8.5 x 200 x 11 x 200 x 1 = 3,740,000 bits
3,740,000 bits / 8 = 467,500 Bytes
467,500 Bytes / 1024 = 456.54 KB (Uncompressed)
While the ability to compress an image file is greatly affected by the complexity of the image, a good rule of thumb is that TIFF Group 4 compression can provide an average compression ratio of 20:1.
Applying TIFF Group 4 compression to the uncompressed image in the example above should result in a compressed file size of approximately 23KB.
Level of Complexity