Tape Measure App
To be able to measure your phone first needs to get some basic data on the space to be able to measure accurately. Move the phone around until the prompt disappears. You can then begin measuring.
Tap the + symbol the set your first point and again to set additional points. You will use the + to start measuring in all applicable measuring tools.
The app offers a number of options for measurement, tap the icon in the lower right corner, this will allow you to access additional measurement tools and switch between them.