What Is PD and Why It Matters
Pupillary distance tells the lab where to position the optical center of each lens. If the PD is off by even 2mm, you are looking through the wrong part of the lens. This causes eye strain, headaches, and sometimes double vision. For prescriptions stronger than +/- 4.00, PD accuracy matters even more because the lens power changes more dramatically away from the center.
Normal adult PD ranges from 54mm to 74mm. The average is about 63mm for men and 60mm for women. Most people are within a few millimeters of these averages.
Method 1: Mirror and Ruler
Stand in front of a mirror with a millimeter ruler. Close your right eye. Align the 0mm mark with the center of your left pupil. Close your left eye, open your right. Read the measurement at the center of your right pupil. That number is your distance PD. Do it three times and average the results. This method is accurate enough for most prescriptions.
Method 2: Friend with a Ruler
Have a friend stand about arm's length away. Look straight ahead past their ear - do not look at them or the ruler. Have them hold the ruler across your brow and measure from the center of one pupil to the center of the other. This is generally more accurate than the mirror method because you are not trying to close one eye.
Method 3: Fytoo's Online Tool
Fytoo has a free PD measurement tool on their website. It uses your webcam and a standard-size card (like a credit card) for scale. Takes about 2 minutes. This is the easiest method and usually accurate within 1mm. If you are ordering progressives or high-index lenses, use this method or get a professional measurement.
Dual PD vs Single PD
Some prescriptions ask for two numbers (right eye PD and left eye PD) instead of one total. Most people's eyes are not perfectly symmetric - one might be 31mm from center, the other 32mm. If you have a dual PD measurement, use it. If you only have single PD, divide by two for the approximate per-eye number. Fytoo accepts both formats.
Distance PD vs Near PD
For reading glasses or progressives, you need your near PD, which is about 3mm smaller than your distance PD. When you focus on something close, your eyes converge - they turn slightly inward. A distance PD of 63mm typically converts to a near PD of about 60mm. If you order reading glasses using your distance PD, the optical centers will be too far apart and you will feel eye strain when reading.
Fytoo and most online retailers automatically adjust for near PD when you select reading or progressive lenses - just enter your distance PD and they do the math. If you are ordering dedicated single-vision reading glasses from a site that does not auto-adjust, subtract 3mm from your distance PD before entering it.
Common PD Measurement Mistakes
(1) Looking at the ruler instead of past it - your eyes converge when you focus on something close, giving a reading that is 1-2mm too narrow. Always look at a distant object or past the person measuring you. (2) Using a ruler with no millimeter markings - imperial rulers with 1/16-inch increments are not accurate enough; you need a millimeter ruler (1mm = 0.04 inches). (3) Measuring once and calling it good - take 3 measurements and average them. If any measurement differs by more than 1mm from the others, discard it and take 2 more. (4) Forgetting that PD changes - children's PD increases as they grow, and for adults, PD can shift by 1-2mm over decades. A PD measurement from 5 years ago may no longer be accurate. (5) Mixing up dual PD (per-eye) and single PD (total). Normal adult range: 54-74mm total, with most adults falling between 58-68mm. If your measurement is outside 54-74mm, remeasure - it is probably a mistake.
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FAQ
How do I measure my PD if I wear glasses now?
Take your glasses off first. The measurement is from your actual eyes, not from your current lenses. If your current glasses fit well and you know the PD they were made with, you can also just use that number again.
Is the mirror method accurate enough?
For single-vision prescriptions under +/- 4.00, yes. The margin of error with the mirror method is about 1-2mm, which is within tolerance for moderate prescriptions. For strong prescriptions or progressives, use Fytoo's online tool or get a professional measurement.
What if I enter the wrong PD?
Fytoo has a 30-day return policy. If your glasses feel wrong when they arrive, the PD is often the culprit. You can return them and reorder with the correct measurement. The most common symptom of wrong PD is a feeling that your eyes are being pulled sideways or that you cannot focus comfortably.