Pupillary Distance (PD) is a required measurement when ordering new prescription eyewear. Expressed in millimeters, PD ensures the glasses manufacturer properly aligns your vision correction. Knowing your PD allows you to purchase glasses from a wider range of sources, including online retailers. A common tape measure or ruler can accurately determine this measurement for a correct fit.
What Pupillary Distance Is
Pupillary Distance (PD) is the measurement, in millimeters, between the centers of your two pupils. For adults, this distance is fixed. The average PD typically falls between 54 and 74 millimeters.
PD dictates where the optical center of each prescription lens must be placed. Misalignment means the lens will not function as intended. An incorrect PD can cause uncomfortable visual symptoms, such as eye strain, blurred vision, or headaches.
PD is recorded as either binocular or monocular. Binocular PD is a single number representing the total distance between the pupils. Monocular PD (dual PD) is a pair of numbers measuring the distance from the center of each pupil to the bridge of the nose. Monocular measurement is more precise as it accounts for facial asymmetry.
Step-by-Step Guide for Measuring PD
Measuring PD at home requires the help of a friend or family member. Gather a millimeter ruler or flexible tape measure, as millimeters are the standard unit. The person being measured should remove eyewear and focus on a fixed object 10 to 20 feet away, keeping their head level. This distant focus ensures you measure the “distance PD” used for general-purpose glasses.
The helper should position themselves directly in front of the person being measured, ensuring they are at the same eye level. Hold the measuring tool horizontally across the bridge of the nose. Align the zero mark exactly over the center of one pupil, typically starting with the right eye.
Measuring Binocular PD
Without moving the zero mark, read the measurement where the center of the other pupil aligns with the scale. This single number is the binocular PD in millimeters.
Measuring Monocular PD
For monocular PD, measure from the center of the bridge of the nose to the center of the right pupil, and then repeat for the left pupil. Record the two numbers separately (e.g., 32/30), listing the right eye (OD) first. The two monocular measurements should add up to the total binocular PD.
Tips for Accuracy and Application
To improve accuracy, repeat the measurement process two or three times. Taking multiple readings helps account for minor movements or errors, allowing you to calculate a reliable average. Ensure the helper keeps the ruler or tape measure straight and parallel to the floor during each attempt.
When submitting your prescription, use the PD number you measured. If the retailer requires monocular PD but you only have the binocular number, divide the binocular number by two. This assumes perfect facial symmetry (e.g., 64 mm binocular PD converts to 32/32 monocular PD).
If ordering reading glasses, the PD for near vision is slightly smaller than the distance PD because eyes naturally converge when focusing close up. Subtract about 3 millimeters from your measured distance PD to approximate your near PD (e.g., 64 mm distance PD becomes 61 mm for reading glasses).