Pupillary Distance (PD) Calculator
Measure your pupillary distance for accurate eyeglass fitting.
Understanding Pupillary Distance (PD)
Pupillary Distance (PD), also known as interpupillary distance, is the measurement between the centers of your pupils. This measurement is crucial for opticians and optometrists when fitting eyeglasses. The correct PD ensures that the optical center of each lens aligns perfectly with the center of your pupil. This alignment is vital for clear vision, comfort, and to prevent visual strain or headaches.
Eyeglass lenses are ground to have a specific optical center, which is the point where light passes through without distortion. When your glasses are made, the frame's bridge and temple arms position these optical centers in front of your pupils. If the optical centers do not align with your pupils, you might experience:
- Blurred or distorted vision
- Eyestrain
- Headaches
- Double vision
- Discomfort
How Pupillary Distance is Measured
Pupillary distance can be measured in a few ways:
- By an Optician/Optometrist: This is the most accurate method. They use a specialized ruler called a pupillometer or a digital device.
- Using a Ruler (Self-Measurement): While less precise than professional methods, it can provide a good estimate. Stand about 8 inches (20 cm) from a mirror or have a friend measure for you. Hold a ruler against your face, just below your eyes. Close your right eye and align the '0' mark on the ruler with the center of your left pupil. Then, open your right eye and close your left. The measurement on the ruler that aligns with the center of your right pupil is your PD. Repeat a few times to ensure consistency.
Calculating PD Segments for Eyeglass Fitting
When ordering glasses, especially online, you may be asked for your monocular PD (the distance from the center of your pupil to the bridge of your nose) or binocular PD (the total distance between the centers of your pupils). Our calculator helps break down the total PD into segments, which is often needed when lenses are not centered perfectly in the frame or for progressive/bifocal lenses.
In this calculator, we're demonstrating how to determine the distance from the center of each pupil to the optical center of the lens, assuming the lens's optical center is intended to be aligned with the pupil's center in a well-fitted frame.
The Math Behind the Calculation:
The 'Pupillary Distance (PD) Calculator' above takes your total PD and the distances from the pupil center to the outer edges of the lens frames. It calculates the distance from the pupil center to the optical center of the lens for each eye.
Let:
Total PD= The overall distance between the centers of your pupils (in mm).Lens Distance Left= The distance from the center of your left pupil to the point where the lens edge would be if it were perfectly centered on your pupil.Lens Distance Right= The distance from the center of your right pupil to the point where the lens edge would be if it were perfectly centered on your pupil.
The calculation assumes that the frame and lens are designed such that the optical center of the lens is intended to align with the center of the pupil. In a perfectly symmetrical scenario, Lens Distance Left would ideally be equal to Lens Distance Right, and their sum would approximate the Total PD.
However, in practice, the "distance from pupil center to lens edge" measurements provided by opticians or derived from frame measurements can vary. This calculator uses the provided values to report these distances directly, as they are often the key metrics needed for custom lens grinding or frame adjustments where the lens needs to be cut to a specific size relative to the pupil.
Example: If your Total PD is 62mm, and you are told that the distance from the center of your left pupil to the edge of where the lens will be is 25mm, and for the right eye it's 30mm, this suggests the optical center for the left lens should be 25mm from the outer temporal edge of where the lens will sit, and 30mm for the right. The sum (25 + 30 = 55mm) might not exactly equal your total PD (62mm) due to frame geometry, lens curvature, or how the measurement was taken (e.g., measuring to the frame edge vs. the lens edge). The critical values for an optician are often these individual distances.