[MetroMPG]

You don't necessarily need to put a ruler in the image; you just need to know the measurement of something of the image.

Then you find the measurement of that item in pixels with the ruler tool, or the select tool (with the "info" window open), and do the math.

EG. let's say your door handle is actually 15 cm across, and in the image it's 90 pixels wide. Pixels per cm = 90/15 = 6.

Is that what you were after?