Arithmetic
Scan size calculator
The settings on a scanner are chosen once and regretted for years. This shows what each combination costs before you feed forty pages through.
Change the DPI and watch what happens. It is not linear.
Estimate
Doubling the DPI quadruples the file
This is the thing that catches everybody. Resolution applies in two directions, so going from 300 to 600 DPI does not double the size, it multiplies it by four. A twenty page colour scan that would have been 12 megabytes becomes 48, and for ordinary printed text nothing is gained, because 300 DPI already resolves detail finer than the print itself.
Colour mode has a similar effect in the other direction. Greyscale stores one channel instead of three; black and white stores one bit per pixel and then compresses runs of white extremely well. A text document scanned in black and white at 300 DPI is a fraction of the same page in colour, and for a signed form it looks the same.
What to actually choose
Printed text or a form: 300 DPI, black and white or greyscale. A document with photographs or a coloured letterhead: 300 DPI colour. An old photograph you want to preserve: 600 DPI colour, and accept the size. Anything above 600 for a document is storage you will never get value from.
Common questions
My scanner produced a much smaller file than this. Why?
Good, that means it applied compression well, which the estimate accounts for conservatively. Mostly white text pages compress far better than average.
Should I scan to PDF or to JPEG?
PDF for anything multi-page, because it keeps the document together and the page order intact. Individual images only when you want to work on them.
I already scanned it and it is too big.
Then compress it to a target. Rescanning at a lower DPI gives a better result, but only if the pages are still to hand.
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