Pupillary distance (PD) is the space between the centers of your two pupils, measured in millimeters. The average adult PD is about 63 mm, with most people falling between 50 and 70 mm. You need this number when ordering glasses online, because it tells the lab where to place the optical center of each lens so it lines up with your eye. Getting it wrong can cause eyestrain, blurry vision, or headaches. The good news: measuring it yourself takes about two minutes with a millimeter ruler and a mirror.
What You Need
A flat ruler with millimeter markings is the only essential tool. Most standard rulers have them, but if yours only shows inches, it won’t work. You can also print a PD ruler from an online eyewear retailer. If you go that route, verify the printed scale is accurate by holding it against a standard ruler or a credit card (which is always 85.6 mm wide). A mirror is necessary if you’re measuring alone. If someone can help you, the mirror is optional.
Measuring Your Own PD With a Mirror
Stand about 8 to 12 inches from a mirror and hold the ruler flat against your brow, resting it on the bridge of your nose. Line up the zero mark directly above the center of one pupil. Keep your head straight and look at your own eyes in the mirror, not at the ruler. Without moving your head, read the millimeter mark that falls directly above the center of your other pupil. That number is your binocular PD.
The trickiest part is resisting the urge to shift your gaze toward the ruler. Your eyes need to stay focused straight ahead at their own reflection. If you glance sideways to read the number, your pupils move and the measurement shifts. It helps to close one eye at a time: close your right eye while you align the zero over your left pupil, then close your left eye and read the number over your right pupil.
Repeat at least three times and write down each result. If the numbers vary by more than a millimeter or two, throw out any outlier and average the rest. A consistent measurement within 1 mm is what you’re aiming for.
Having Someone Else Measure You
This method is generally more accurate because the person reading the ruler can see both your pupils without any parallax issues. Stand facing your partner at about arm’s length. Place the ruler on the bridge of your nose. Close your left eye, and have your partner align the 0 mm mark over the center of your right pupil. Then close your right eye and open your left. Your partner reads the millimeter mark over your left pupil’s center. That’s your PD.
The reason you alternate which eye is open matters: it keeps whichever eye is being measured looking straight ahead rather than drifting inward. As with the mirror method, take three measurements and average them for the most reliable number.
Binocular vs. Monocular PD
The single number you get from the methods above is your binocular PD, the total distance from one pupil to the other. Most online glasses retailers accept this. Some ask for a monocular PD instead, which is two separate numbers: the distance from the bridge of your nose to each pupil. These two numbers won’t necessarily be equal, because most people’s faces aren’t perfectly symmetrical.
To get your monocular PD, use the same ruler technique but measure from the center of each pupil to the center of the bridge of your nose (roughly the midpoint between your eyes). You’ll end up with something like 31 mm for the right eye and 32 mm for the left. If a form asks for “right PD” and “left PD,” those are your monocular values. Added together, they should equal your binocular PD.
Smartphone Apps
Several iPhone apps now measure PD using the front-facing camera. A 2023 study published in Investigative Ophthalmology & Visual Science tested two popular apps against a handheld ruler and a professional pupillometer across 178 people. The apps produced results within about 0.7 mm of the professional tools on average, which is well within the range that matters for most prescriptions.
Where the apps actually outperformed the manual methods was consistency. When the same person was measured multiple times, the apps varied by only about 0.2 to 0.3 mm between sessions, while the handheld ruler varied by about 0.75 mm and even the pupillometer varied by 0.6 mm. So if you struggle with the ruler method or keep getting different numbers, an app can be a reliable alternative. Most require you to hold a reference object (like a credit card) near your face so the software can calibrate the scale.
What Counts as a Normal Range
Most adults measure between 50 and 70 mm, with 63 mm as the average. A small number of people fall outside that window, anywhere from 45 mm to 80 mm. Your PD stabilizes once you’re done growing, typically in your late teens, and stays the same for the rest of your life. Children’s PD changes as their face develops, which is one reason kids’ prescriptions need updating more frequently.
If your measurement falls at the extreme ends of the range, double-check it with a second method or a second person. An unusual PD isn’t a problem on its own. It just means the lab needs the correct number so your lenses are ground with the optical centers in the right position.
Common Mistakes to Avoid
- Tilting the ruler. If the ruler isn’t flat against your brow and parallel to the floor, you’ll read a shorter distance than your actual PD. Keep it level.
- Moving your eyes to read the number. Your gaze should stay fixed straight ahead. Any sideways glance shifts your pupil and skews the result.
- Rounding to the nearest 5 mm. PD matters to the millimeter. If the mark falls between 62 and 63, record 62.5 rather than rounding to 65.
- Measuring only once. A single reading can easily be off by 1 to 2 mm. Three measurements averaged together give you a much more dependable number.
- Standing too far from the mirror. The farther away you are, the harder it is to see the fine millimeter markings and your pupil centers clearly. Eight to twelve inches is the sweet spot.
Where Your PD Goes on a Glasses Order
When ordering glasses online, you’ll see a field for PD alongside your sphere, cylinder, and axis numbers from your prescription. Some retailers have a single field for binocular PD. Others split it into right and left monocular values. If you only have the binocular number and the form asks for monocular, dividing by two is a reasonable approximation, though measuring each side individually is more precise if your face is noticeably asymmetric.
Your eye doctor may or may not include PD on your printed prescription. In some states, they’re required to; in others, they aren’t. If it’s listed, use that number. If it’s not, the at-home methods above will get you close enough for standard single-vision lenses. For progressive or bifocal lenses, where the optical center placement is more critical, having a professional measurement is worth the extra effort.