The process of ordering new eyeglasses, especially online, often requires a measurement that may not be immediately obvious on your prescription slip: Pupillary Distance, or PD. This measurement is the space between the centers of your pupils, usually expressed in millimeters (mm). Without this number, the lenses cannot be correctly aligned in the frames, which is essential for ensuring you receive the clearest and most comfortable vision correction. Locating or obtaining your PD is a practical first step to successfully purchasing new eyewear.
Defining Pupillary Distance
Pupillary distance is the physical measurement between the centers of the pupils in both eyes, ensuring the optical center of the lens lines up perfectly with the center of your pupils for best vision correction. An inaccurate PD measurement, even if off by just a few millimeters, can lead to eye strain, headaches, and distorted vision. The average PD for adults typically falls within the range of 54 mm to 74 mm.
There are two main types of PD measurement: binocular and monocular. Binocular PD is a single number representing the total distance between the centers of both pupils, often used for basic single-vision prescriptions.
Monocular PD, also called dual PD, is a more precise measurement that provides two separate numbers, one for each eye, representing the distance from the center of each pupil to the bridge of the nose (e.g., 32/31). Because most people have slight facial asymmetry, the monocular measurement ensures the lens centers are perfectly positioned. Monocular PD is recommended for higher prescriptions, progressive lenses, or bifocals to achieve the best visual outcome.
Where to Find PD on Your Prescription
Finding your Pupillary Distance on a prescription requires checking for common abbreviations like “PD,” “P.D.,” or “D.P.D.” The measurement is almost always given in millimeters and is often written separately from the main grid listing your Sphere, Cylinder, and Axis values.
The PD may appear as a single number, such as “64,” indicating a binocular distance. Alternatively, you might see two numbers separated by a slash or listed side-by-side (e.g., “32/31” or “32 R / 31 L”), which indicates the monocular PD for your right and left eyes.
If you see two pairs of numbers (e.g., “33.5/32.0” and “33.5/32.0”), the first number in each pair is typically the distance PD, and the second is the near PD used for reading glasses.
If you cannot locate the measurement, it may have been intentionally omitted by the prescribing provider. Some practices may leave it off to encourage patients to purchase eyewear directly from their office. The simplest option is to contact your eye care provider and request the measurement, as they have it on file from your exam.
How to Measure Your Own Pupillary Distance
If obtaining the PD from your eye care provider is not an immediate option, you can take a measurement yourself using a millimeter ruler and a mirror, standing about eight inches away. Align the zero mark of the ruler with the center of one pupil while looking straight ahead.
To find your total binocular PD, read the measurement on the ruler that corresponds to the center of the other pupil. To improve accuracy, close one eye when aligning the zero mark and the other eye when taking the final measurement. Repeat this process two or three times to ensure the number is consistent.
If you prefer the more precise monocular PD, measure the distance from the center of the bridge of your nose to the center of each pupil separately. While a doctor using a pupillometer provides the most precise measurement, a careful self-measurement is a viable alternative for ordering glasses. A friend can also assist by measuring your eyes while you look straight ahead at a distant object.