Estimate storage for scanned documents.
Scan size is driven by resolution squared, so moving from 200 to 400 dpi quadruples the data rather than doubling it. An A4 page at 300 dpi is about 8.7 million pixels; multiplying by bytes per pixel gives the raw size, where bitonal uses an eighth of a byte and full colour three. Compression then divides that, with typical document ratios around ten to fifteen to one.
Raw page size
Raw MB = 8.27 x 11.69 x dpi squared x bytes per pixel / 1048576
Archive size
Total GB = raw MB / compression ratio x pages / 1024
300 dpi is the standard for text documents and is what OCR engines expect. 600 dpi is worth it only for fine print, engineering drawings or archival reproduction.
Twenty-four bit colour stores three bytes per pixel against one for greyscale. Scanning ordinary black text in colour multiplies the archive threefold for no readability gain.