Pupillary Distance (PD) is a precise measurement required when ordering prescription eyewear, especially from online retailers. Since PD is frequently omitted from standard eye prescriptions, acquiring new glasses can be challenging. This measurement ensures your lenses are correctly manufactured and centered to your unique anatomy. Utilizing an existing pair of glasses offers a reliable method to find this measurement at home, providing the necessary data for your next purchase without specialized equipment.
Understanding Pupillary Distance
Pupillary distance defines the measurement, in millimeters, between the centers of your two pupils. This single number is known as binocular PD. An accurate PD guarantees that the optical center of the lens—the point of clearest vision correction—aligns precisely with the center of your pupil. When alignment is incorrect, light rays are improperly refracted, creating a prismatic effect. This forces the eye muscles to compensate, which can lead to visual strain, headaches, and distorted vision. For most adults, the PD measurement falls within the range of 50 to 75 millimeters.
Gathering Necessary Supplies
This at-home measurement requires only a few easily accessible items. You will need the pair of glasses you wish to use, ensuring they fit your face comfortably and correctly. A fine-tipped permanent marker is necessary for marking the lenses without smudging, creating a small, distinct dot on the surface. A metric ruler is required, as PD is universally measured in millimeters. Finally, you will need either a large, well-lit mirror or the assistance of a trusted helper to facilitate the marking process accurately.
Step-by-Step Measurement Using Existing Lenses
Focusing
Begin by putting on your existing glasses, ensuring they sit in the correct, natural position on your face. Focus your eyes on a single point far away, ideally at least 20 feet in the distance. This distant focus ensures your eyes are in their parallel position, which is necessary for obtaining a distance PD measurement. This distance PD is the standard measurement used for most non-reading glasses prescriptions.
Marking the Lenses
Next, mark the center of your pupil onto the lens. If using a mirror, close your left eye and position the marker directly over the center of your right pupil, placing a small dot on the lens surface. Repeat this process for your other eye. If a helper is assisting, they should close one eye to avoid parallax error and mark the center of each pupil sequentially. The goal is to accurately transfer the pupil’s center onto the lens.
Measuring the Distance
After marking both centers, remove the glasses carefully to avoid smearing the dots. You should now have two small dots on the lenses, one for each pupil center. Use your metric ruler to measure the distance between the centers of the two marked dots. Align the zero mark of the ruler with the center of the first dot, and read the millimeter mark that aligns with the center of the second dot. This value is your total binocular pupillary distance.
Ensuring Reliability
To increase reliability, repeat the entire marking and measuring process two or three times. Consistency between the measurements confirms the accuracy of your technique. Averaging the closest results can help mitigate minor errors that occurred during the marking process.
Checking Your Results and Formatting for Ordering
Once the measurement is complete, check the resulting number against the adult range of 50 to 75 millimeters. A number falling far outside this range suggests an error in the measurement process. The method yields a single number, which is the binocular PD (e.g., 62 millimeters).
Some online retailers, particularly for advanced lenses like progressives, may ask for a monocular PD, which is the distance from the center of each pupil to the bridge of the nose. Monocular PD is written as two separate numbers (e.g., 31/31). Since the at-home method provides a binocular PD, you can divide this number by two to get the monocular PD for each eye, such as splitting 62 millimeters into 31 millimeters for the right eye and 31 millimeters for the left eye.
While this DIY method is reliable, it assumes a perfectly symmetrical face, which is uncommon. If you notice that one of your marked dots appears significantly closer to the nose bridge than the other, you may consider measuring the distance from the center of each dot to the center of the nose bridge on the frame to obtain a more precise, but still approximate, monocular PD. Self-measurements carry a small margin of error compared to those taken professionally with a specialized instrument called a pupillometer.