Pupillary Distance (PD) is the measurement in millimeters between the centers of your pupils. This number is a fundamental component of your eyeglass prescription. An accurate PD measurement is necessary because it ensures the optical center of your corrective lenses aligns precisely with the center of your pupils. Misalignment, even slight, can cause the wearer to experience distorted vision, eye strain, or headaches, ultimately compromising the quality of visual correction.
Getting Measured by an Eye Care Professional
Seeking a measurement from a qualified eye care professional is the most accurate way to determine your Pupillary Distance. Optometrists and opticians typically take this measurement during a comprehensive eye examination using a specialized instrument called a pupillometer. This non-invasive digital device provides a highly precise measurement, often down to a tenth of a millimeter. This method is the gold standard for accuracy, particularly for individuals with strong or complex prescriptions.
The PD measurement is not always automatically included on your written prescription due to varying state regulations and professional practices. If you plan to order glasses from a source other than your eye doctor’s office, you must specifically request that they provide you with your PD number. Many large retail optical chains also offer free or low-cost PD measurements as a courtesy service. The professional environment ensures the measurement is taken with your eyes focused on a distant object, which provides the most appropriate PD for general distance vision.
How to Measure Your Pupillary Distance at Home
Obtaining an accurate PD measurement at home requires only a millimeter ruler and a mirror or a helper.
Using a Ruler and Mirror
For the mirror method, stand approximately eight inches away and hold the ruler horizontally against your brow. Ensure the zero mark is centered over the pupil of one eye. While keeping your head perfectly still, read the millimetre marking that aligns with the center of your other pupil, which gives you your total distance PD. The key is repeating the measurement multiple times to ensure consistency.
Using a Helper
If you have a helper, the process can be easier for distance PD. Stand facing your helper about an arm’s length away and instruct them to hold the millimeter ruler flat against your brow. They should align the zero mark with the center of your right pupil while you look straight ahead at an object in the distance. The millimetre mark that aligns with the center of your left pupil is your measurement.
Digital Tools
Various digital tools and smartphone applications are also available that use your device’s camera and a standard reference item, like a credit card, to calculate your PD. These apps are convenient and can provide a relatively good estimate, though they may not match the precision of a professional pupillometer. Whichever method you choose, it is advisable to take the measurement three to four times and use the average number to minimize any human error.
Interpreting Your PD Number
Your PD will typically fall within the range of 54 millimeters to 74 millimeters for adults. This single number represents the total distance between the centers of both pupils and is known as the binocular or single PD.
Single vs. Dual PD
A more precise measurement is the dual or monocular PD, which is preferred for higher prescriptions, progressive lenses, or those with facial asymmetry. This method yields two separate numbers, such as 32/30. These numbers represent the distance from the center of the nose bridge to the center of the right pupil and the left pupil, respectively. Always use the dual PD if it is available, as it allows for a more customized lens placement within the frame.
Distance vs. Near PD
The PD number you obtain is usually your distance PD, which is the measurement taken when your eyes are focused far away. If you are ordering reading glasses, the required PD is slightly smaller because your eyes naturally converge when focusing on a near object. For near-vision tasks, calculate your near PD by subtracting 3 millimeters from your single distance PD, or 1.5 millimeters from each of your dual PD numbers.