Don’t hang up your carpenter’s square quite yet; it will still be useful in the layout of your treads.
But today’s smartphones have mini gyroscopes called MEMS (microelectromechanical system) gyros (that’s “JIGH-ro, not the Greek food GEE-ro).
These sensors allow very accurate measurement of the phone’s orientation to a plumb line.
Step 2: Hold the phone with long axis along the string — as parallel as possible, please.
Step 3: Read degrees and, if needed, convert to XX on 12 pitch by using the formula 12*tan(degrees). In this photo, 35.3 degrees on the level equals a 8.49 to 12 pitch.