​How does DPI affect file size?

This is a common question we hear at Large Document Solutions.  We rent and sell large format scanners for archival and digitizing projects.  Although disk space has become very cheap, companies are still concerned with disk space usage.

A test was completed to estimate file sizes.  Let’s take a small format example.  Our sample is an 8 ½” x 11” standard sheet of paper or ANSI A size.  Our scan was saved as a 24 bit color PDF file.  The results are as follows:
 

DPI	Size (KB)
200	461
300	1153
400	1942
500	3780

The results showed that for each 100 dot per inch scanned the file size doubled and in straight line proportion.  The conclusion: If you do not need a high density of dots in your archive scan, don’t.  If file size is not an issue but density is important, go ahead and scan at 300-400 DPI.  Anything denser than 400 DPI there is no visual difference unless a requirement is to zoom into the drawing.  The more dots per inch, the better image will be displayed on a zoomed in look at the image.

The standard large format size is an Arch D size or 24” x 36” size document.  This is 9X the size of the sample 8 ½’ x 11” used.  File sizes also vary with how much data is on the page.

When this question is asked, there are many factors in contributing to the ultimate file.  The best way to estimate your file size is to take an average drawing, scan it using the method that optimally will work for your needs then decide if this size will work in your environment.  If not, modify your scan parameters to fit your needs.