As pupils are quite dark, it is sometimes difficult to know where the exact centre is. One method which ensures you will get the right measurement is by using the border between the white part of your eye, (sclera) and the black pupil.
Close one eye and line up the Zero (0) of the ruler to either the inner or outer pupil edge, remembering to always have one eye closed, and read across to the opposite pupil edge.
Always measure outside pupil edge to inside, or inside to outside, never outside to outside, which would mean that the PD measurement would be too large, or inside to inside, which would mean the PD measurement would be too small.
As a routine, PD is not added to your prescription, but you can request it when you get a copy of your prescription.
Don’t worry if you do not have this as its easy to measure.