Pupillary distance (PD) is the distance in millimeters between the centers of your two pupils, and you can measure it at home with a millimeter ruler and a mirror. Most adults fall between 54 and 74 mm, while children typically range from 43 to 58 mm. Getting this number right matters because it tells the lens maker where to place the optical center of each lens so it lines up with your eyes.
Why PD Accuracy Matters
When the optical centers of your lenses don’t sit directly over your pupils, the misalignment creates unwanted prismatic effects. Your eyes are forced to work harder to merge two slightly shifted images into one, which can cause eye strain, headaches, or in more extreme cases, double vision. One eye may even start suppressing its image to compensate.
How much error you can tolerate depends entirely on how strong your prescription is. For a mild prescription, being off by 2 mm might not cause noticeable problems. For a strong prescription (say, above 5 or 6 diopters), even 1 mm of error in each eye can produce roughly 1.5 diopters of induced prism, enough to create a pulling sensation, shadows, or double vision. Industry standards from ANSI Z80.1 reflect this: progressive lenses must be within 1 mm of the specified monocular PD, while standard single vision lenses get a slightly wider tolerance of 2.5 mm.
How to Measure PD by Yourself
You need a millimeter ruler (not inches) and a well-lit mirror. Stand about 8 inches from the mirror and hold the ruler horizontally across the bridge of your nose, resting it against your brow for stability. Align the zero mark directly over the center of your left pupil. Then close or cover your left eye and read the millimeter mark that falls over the center of your right pupil. That number is your PD.
Look straight ahead into the mirror the entire time, not down at the ruler. Your gaze should be relaxed and focused on a point in the distance (your own eyes in the mirror work fine). Repeat the measurement three or four times and use the number that comes up most consistently. If your readings vary by more than a millimeter, your ruler may be shifting between attempts, so try pressing it more firmly against your forehead.
How to Measure PD With a Partner
Having someone else measure you can be more accurate because you don’t have to look at a ruler and a mirror simultaneously. Stand facing your partner at about arm’s length. Look straight ahead at something behind them, not at their face, so your eyes stay relaxed and parallel.
Your partner places the ruler across the bridge of your nose. You close your right eye, and your partner aligns the 0 mm mark over the center of your left pupil. Then you close your left eye and open the right. Without moving the ruler, your partner reads the millimeter mark over the center of your right pupil. That reading is your binocular PD. As with the mirror method, repeat a few times to confirm consistency.
Binocular PD vs. Monocular PD
A binocular PD is a single number, like 63 mm. A monocular PD is two separate numbers, one for each eye, measured from the center of the bridge of your nose to each pupil individually (for example, 31.5 mm left, 31.5 mm right). Most people’s faces aren’t perfectly symmetrical, so the two monocular values often differ by half a millimeter or more.
For single vision lenses with a mild prescription, a binocular PD is usually sufficient. But if you’re ordering progressive lenses or have a strong prescription, a monocular measurement is important. Progressive lenses have a narrow corridor of correct focus, and even small horizontal misalignment can push you outside that corridor, blurring your vision at certain distances. If your prescription is above roughly 4 or 5 diopters, ask for monocular PD values or measure them yourself by noting the ruler reading at the center of your nose for each eye separately.
Distance PD vs. Near PD
Your pupils move closer together when you look at something nearby. This convergence shrinks your PD by about 3 mm compared to looking at a distant object. So if your distance PD is 64 mm, your near PD is approximately 61 mm.
This distinction matters mainly for reading glasses or the near portion of bifocals and progressives. When you order single vision reading glasses, the optical centers should be set to your near PD so the lenses align with where your eyes actually point while reading. Most online retailers will ask whether you want distance or near PD, or they’ll ask what the glasses are for and adjust accordingly. If they only ask for one PD number and you’re ordering reading glasses, subtract 3 mm from your distance measurement.
Getting Your PD From Your Eye Doctor
Eye care professionals typically measure PD with a device called a pupillometer, which uses light reflections off your corneas to pinpoint pupil position digitally. This is generally the most reliable method, especially for monocular measurements.
Your PD is not always included on your prescription automatically. The FTC’s Eyeglass Rule requires prescribers to release your prescription so you can shop wherever you want, but it doesn’t specifically mandate that PD be included. Some states do require it. If your provider took the measurement, the FTC encourages them to share it with you. Simply ask for it at the end of your exam. If they didn’t measure it, or if you’d rather not go back for it, the at-home methods above will get you a reliable number for most prescriptions.
Tips for a More Accurate Measurement
- Use good lighting. Your pupils constrict in bright light, making the center easier to identify. Dim lighting causes pupils to dilate, which makes the edges less defined and the center harder to pinpoint.
- Keep the ruler level. Tilting it even slightly across your face can add or subtract a millimeter. Resting it against your brow ridge helps keep it horizontal.
- Measure multiple times. Three to five readings that agree within 1 mm give you a trustworthy number. If you’re getting a spread of 3 mm or more, something about your technique is inconsistent.
- Wear your contacts if you have them. If you normally wear contacts and are measuring PD for a backup pair of glasses, it doesn’t matter. But don’t wear old glasses while measuring, because the lenses can shift the apparent position of your pupils.
- Consider your prescription strength. If your prescription is above 5 diopters, a professional measurement with a pupillometer is worth the effort. The margin for error shrinks significantly at higher powers, and the consequences of getting it wrong are more noticeable.