@lostbrane You need to use a separate app called Measure. It opens up automatically to a photo app. You let it calibrate, then you put the dot on one end and click the +, then you move the dot to the other end and click on + again for it to take the measurement for you.
It will be useful for lots of stuff.
For accuracy, test measure apps against an actual ruler from different distances and angles. Some co-workers and I were playing with a bunch of these apps on different phones with widely varying results from "slightly more accurate than a carpenter's guesstimate" to utterly frigging useless. The iPhone app is one of the better ones.
I wouldn't quote a sale based on numbers from any of them.
For home and hobby I'll be sticking with the tried and true guesstimates and rulers.
Curious to know if anyone finds something useful on Android (I sure didn't).