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In document capture, file size is one of the most important factors to consider, due to it's impact on storage and bandwidth requirements. Getting the smallest possible file size will be crucial for the success of any document capture project.

Below is a theoretical overview on how to calculate the file sizes.  

1. Calculating the file size of a scanned image (uncompressed)

Uncompressed file size = Resolution in dpi (horizontal ) x Resolution (vertical) x Height (inches) x width (inches) x color depth

Example 1 - scanning an A4 page in 200 dpi in B&W

= 200 x 200 x 8.27 x 11.69 x 1

= 3,867,052 bits

= 483,381 bytes

= 472 Kb

Approximate size of an uncompressed A4 image scanned @ 200 dpi / B&W = 500 Kb / 0.5 Mb

Example 2 - scanning an A4 page in 200 dpi in grayscale (8 bits / 256 shades of grey)

= 200 x 200 x 8.27 x 11.69 x 8 bits

= 3.68 Mb

Grayscale image size = 8 times of B&W equivalent

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2. How the file size will change when the resolution is changed

Example 1 - scanning an A4 page in 400 dpi in B&W

= 400 x 400 x 8.27 x 11.69 x 1

= 15,468,208 bits

= 1,933,526 bytes

= 1,888 Kb

Doubling the resolution will quadruple the file size

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3. Calculating the file size of a scanned image (compressed)

Compressed file size = uncompressed file size x compression ratio

Compression ratios (approx.)

CCITTG3 (Group 3)   = 1/10

CCITTG3 (Group 3)   = 1/20

JPEG                           = 1/100

Example 1 - scanning an A4 page in 200 dpi in B&W and compressing with CCITT G3 compression

= 472 Kb * (1/10)

= Approx. 50 Kb

Example 2 - scanning an A4 page in 200 dpi in B&W and compressing with CCITT G4 compression

= 472 Kb * (1/20)

= Approx. 25 Kb

So assuming ideal conditions we should be able to get an A4 page scanned at 200 dpi, B&W in to a 25 Kb TIFF file.