Love It!

Recommended for folks who are comfortable with a little geometry and optics. The credit card has to be as close to parallel with the camera as possible, and centered as well as possible in the frame. Light conditions also have to be reasonably good. Once all your ducks are in a row, you can get a pretty good measurement using only an iPhone and a credit card. Rots a ruck!!

Scot Perry about RulerPhone - Photo Measuring, v4.0