Pupillary distance (PD) is the measurement, typically in millimeters, between the centers of your pupils. This measurement is necessary when ordering prescription eyeglasses, especially from online retailers. An accurate PD ensures the optical center of each lens aligns precisely with the center of your eye for the clearest vision. Misaligned lenses, even by a millimeter or two, can cause visual discomfort, eye strain, headaches, and distorted sight.
Preparing the Ruler and Understanding PD Types
Before beginning, confirm your ruler is marked in millimeters (mm), the universal unit for pupillary distance. The average adult PD generally falls between 54 mm and 74 mm. Ensure you have adequate, consistent lighting and maintain a neutral, upright posture with your head straight and eyes level.
PD measurements are recorded in two formats: Single PD or Dual PD. Single PD (binocular PD) is a single number representing the total distance between the centers of both pupils. This is the most common format and works well for standard single-vision lenses.
Dual PD (monocular PD) is a pair of numbers providing the distance from the bridge of the nose to the center of each pupil individually. This format acknowledges that the face is rarely perfectly symmetrical. Dual PD is considered more precise and is preferred for bifocal, progressive, or high-prescription lenses.
The Assisted Measurement Method
Having a helper measure your PD is the most accurate at-home method, reducing potential self-measurement errors. The subject should sit or stand upright and focus their gaze on a stationary object 10 to 20 feet away. Focusing on a distant point ensures the eyes are relaxed and in the proper position for the distance PD measurement required for everyday glasses.
The helper should position themselves directly in front of the subject, ensuring their eyes are on the same horizontal level. They should place the millimeter ruler horizontally across the subject’s face, resting it gently against the browline. For a Single PD measurement, the helper must align the ruler’s zero mark (0 mm) directly over the center of the subject’s right pupil.
Without moving the ruler, the helper reads the millimeter marking that aligns with the center of the subject’s left pupil. This single number is the total PD measurement.
Measuring Dual PD
To obtain the more precise Dual PD, the helper instead aligns the zero mark with the center of the subject’s nose bridge. They then measure the distance from the nose bridge to the center of the right pupil, and separately measure the distance from the nose bridge to the center of the left pupil, resulting in two distinct numbers.
The helper should repeat the measurement process several times to ensure consistency and minimize the chance of a reading error. It is important for the helper to keep their head still and read the ruler straight-on, while the subject must maintain their distant focus and avoid shifting their gaze.
Measuring Your PD Without Assistance (Self-Measurement)
Measuring your own PD requires a mirror and is a viable alternative if assistance is unavailable, but it demands careful technique. Stand approximately arm’s length away from a mirror, maintaining a steady, head-on view of your reflection. Hold the millimeter ruler firmly against your brow or forehead, keeping it level and straight across your face.
To begin the process, you must close your right eye and use your left eye to align the zero mark (0 mm) on the ruler with the center of your left pupil. Holding the ruler perfectly still against your face, you then switch eyes by opening your right eye and closing your left eye. This eye-switching technique is necessary because you cannot look directly at the ruler’s markings and your pupils simultaneously.
Next, you must read the millimeter number that aligns with the center of your right pupil, and this number represents your Single PD. A common challenge in self-measurement is parallax error, which occurs when you view the ruler at an angle instead of straight-on, causing the measurement to appear inaccurate. To minimize this error, ensure the ruler remains perfectly still and that your eye is looking directly at the scale when reading the number.
It is highly recommended that you repeat the entire self-measurement process at least three times, recording each result. Taking the average of these three measurements can help correct for slight movements or observational inaccuracies. If you are aiming for a Dual PD, you will instead align the zero mark with the center of your nose bridge and measure to each pupil separately, using the same eye-closing and switching technique.