the Pythagorean calculator does half the work, but need to change input fields to "Pixels X" and "Pixels Y" or similar input fields, and a 3rd input field - diagonal LCD measurement, to divide the Pythagorean result by... here is an example of the formula

http://www.ehow.com/how_7794090_calculate-pixel-density.html

I took a look at the code, but couldn't figure out where the actual calculations were performed.

http://wordpress.org/extend/plugins/calculatorpro-calculators/