Measuring dimensions inside photos

I have some photos in which I would like to calculate some ratios of the dimensions of objects, by simply having a measuring tool. For example, I would like to measure the ratio of a door by dragging cursor point to one side of the door and then another point on the other side of the door. This woul give me a width measurement. Then I would do the same for the height of the door, which would give me 2 figures to calculate the width/height ratio.
Any idea of a way to enable me to do this?
Thanks!
Jamie