Measurement tool?

I am just starting to explore 3d printing (if you are unfamiliar with 3d printing,you should googel it
and for those how already did this might know that the scaling is wary important.
So now I am looking for a tool that can tell me how thin/thick my model is.Something like a ruler tool would be perfect.
Thanks in advance:)

Blender has Unit setting now under Scene setting. You can set it to Metric or Imperial. And selected edge can display its length by turning on the “Edge Length” in Properties bar. But, I am not sure whether 1 meter in Blender is 1 meter in 3D printing device.

Not sure if 3D View: Measure Panel in User Prefs add-ons could aid you, as far as thick/thin but it is there to try if enabled.

@risix:Thanks so much,this will help a lot!!!

@chip4brains:Also thanks to you!The add on dose not calculate the edge length (as far as I can see,maybe I am rwong) but rather calculates the m^2