MeasureBot calculates the height of objects in the real world using the sensors on your device. Depending on how well the sensors perform in your device, it can calculate the height of a table for example to an accuracy of just a couple of centimetres with just a few waves of your arm. Please note that it may perform inaccurately on devices that have less accurate sensors, but if you are running on a newish device then you should be good to go.