Pupillary Distance (PD), also known as interpupillary distance (IPD), is the precise measurement, in millimeters, between the centers of your two pupils. This measurement is fundamental for manufacturing prescription eyewear because it determines the exact spot on the lens where the optical center must be placed. Aligning the optical center precisely with the pupil’s center ensures that you are looking through the most focused and clearest part of the lens. A correct PD is necessary to prevent visual discomfort, eye strain, or headaches that can result from misaligned lenses.
Locating Pupillary Distance on the Prescription
On your eyeglass prescription, the Pupillary Distance is typically listed under the abbreviation “PD”. This number is usually separate from the detailed lens power information, which includes Sphere (SPH) and Cylinder (CYL). It might be handwritten by the practitioner or printed in a dedicated field on the form.
The measurement often appears as a number between 54 and 74 millimeters. Unlike the prescription for the lens power, which is legally required to be released to the patient, the rules regarding the release of the PD measurement vary by region and sometimes by the clinic’s internal policy. Historically, some eye care providers have omitted the PD from the written prescription to encourage the patient to purchase their glasses directly from the clinic.
If you cannot find the PD clearly marked on your document, you should contact your eye care provider and request it. While laws concerning the mandatory release of spectacle PD may not be as strict as those for contact lens prescriptions, most practitioners will provide the measurement upon request. This number is necessary if you plan to order prescription eyeglasses from an online retailer.
Interpreting Single vs. Dual PD Measurements
The Pupillary Distance will be presented in one of two formats: a single number or two separate numbers. The single PD, also called binocular PD, is the total distance between the center of your right pupil and the center of your left pupil. This results in one number, for example, “62 mm,” and is the most common measurement used for standard, single-vision lenses.
The dual PD, sometimes called monocular PD, is a more precise measurement that accounts for facial asymmetry. This format provides two numbers, representing the distance from the center of the nose bridge to the center of each pupil individually. It is typically written as two numbers separated by a slash, such as “32/30,” where the first number is for the right eye (OD) and the second is for the left eye (OS).
Dual PD is more important for individuals with higher prescriptions or those ordering complex lenses, such as progressive or bifocal types. If you have a single PD number but need to enter a dual PD for an order, you can simply divide the single number by two, which works well for most single-vision prescriptions.
Simple Methods for Measuring PD Yourself
If your prescription lacks the PD, you can obtain a reasonably accurate measurement at home using a millimeter ruler. One common method involves standing approximately eight to twelve inches away from a mirror in a well-lit area. You should hold the ruler horizontally across your brow line and align the zero mark with the center of your right pupil.
Look straight ahead and read the millimeter mark that lines up with the center of your left pupil. This number represents your total, or single, Pupillary Distance. It is advisable to repeat the measurement two or three times to ensure consistency, as this self-measurement technique is generally best for determining your distance vision PD.
Alternatively, having a helper measure your PD can improve accuracy by minimizing head movement. The person being measured should look straight ahead at a distant object (at least 10 to 20 feet away). The helper then aligns the ruler’s zero mark with the center of one pupil and measures the distance to the center of the other pupil.
To obtain a dual PD, the helper can align the zero mark with the center of the nose bridge and measure the distance to the center of the right pupil, then repeat the process for the left pupil. If you are ordering glasses for reading or near work, your near PD will be required, which is typically three to four millimeters less than your distance PD.