Pupilliary distance (PD) is a measurement of the distance between your pupils. The PD is required so that we can make your glasses with the centre of the lens sitting directly in front of your pupil. If you don't know your PD, stand at an arm's-length away from a mirror and place a ruler against your eyebrows. If you use your glasses for distance: Looking straight ahead, align the ruler’s zero to the centre of your left pupil and read the millimetre line that is aligned with the centre of your right pupil. If you use your glasses for reading: Follow the above but minus 2mm from the distance number. You may want to take a few measurements to get a consistent number.