Pupillary Distance (PD) is the measurement in millimeters between the centers of the pupils in both eyes. This precise distance is fundamental for manufacturing prescription eyewear because it ensures the optical center of each lens aligns perfectly with the wearer’s line of sight. An accurate PD is necessary for clear, comfortable vision, especially when ordering new glasses from an online retailer. If this measurement is incorrect, the resulting misalignment can lead to visual discomfort, headaches, and eye strain.
Understanding Pupillary Distance and Tools
PD measurements are categorized into two types: Single (or Total) PD, and Dual (or Monocular) PD. Single PD is one number representing the full distance between the centers of the pupils, often used for simple prescriptions. Dual PD uses two numbers that measure the distance from the center of the nose bridge to the center of each pupil separately (e.g., 32/31). This split measurement is preferred for greater precision, especially for higher prescriptions or progressive lenses, as it accounts for potential facial asymmetry. To measure at home, a simple millimeter ruler is required, though a specialized PD ruler with a nasal notch offers easier alignment.
Measuring Another Person (The Standard Method)
Measuring another person is the most reliable method for obtaining an accurate PD measurement. The subject should focus on a distant object (10 to 20 feet away). This distant focus ensures the eyes are in a relaxed, parallel state, necessary for a distance PD measurement. The measurer should sit opposite the subject, ensuring their eyes are at the same height to prevent parallax, the apparent shift in position.
To measure the Single PD, hold the millimeter ruler horizontally across the subject’s face, resting it against the brow line for stability. Align the ruler’s zero mark precisely with the center of the subject’s right pupil. Note the millimeter mark that aligns with the center of the subject’s left pupil.
To obtain the Dual PD, use the center of the subject’s nose bridge as the starting point. Align the ruler’s zero mark with the nose bridge center, and record the distance to the center of the right pupil. Then, read the distance to the center of the left pupil. This results in two separate numbers (e.g., 33mm/32mm), which is the preferred format for modern lens technology.
Measuring Yourself Using a Mirror
Measuring your own PD requires a mirror and introduces potential error due to the ruler’s proximity. Stand 8 to 12 inches away from a mirror in a well-lit area and hold the millimeter ruler flat against your forehead just above your eyes. The ruler must be horizontal and stable to ensure a reliable reading.
To counteract parallax error, you must alternately close one eye while taking each measurement. For Single PD, close your right eye and align the ruler’s zero mark with the center of your left pupil. Without moving the ruler, open your right eye, close your left eye, and note the millimeter mark that lines up with the center of your right pupil.
For the Dual PD, align the ruler’s zero point with the center of your nose bridge. Read the measurement to the center of the right pupil, and repeat the process to find the distance to the center of the left pupil. Maintaining a steady head position prevents the ruler from shifting and minimizes inaccuracies.
Verifying Accuracy and Recording the Result
To ensure reliability, repeat the measurement process at least three times. Averaging the results helps mitigate minor errors introduced by ruler movements or shifts in eye focus. For adults, the distance PD typically falls within a range of 54 to 74 millimeters.
The final result should be recorded in the format requested by the eyewear provider. If you measured the total distance, the result is a single number (e.g., 63). If you used the Dual PD method, the measurement is recorded as two separate numbers, with the right eye (OD) distance listed first (e.g., R 32 / L 31). If your prescription includes prism correction or high power, seeking professional verification is advisable.