You can measure your pupillary distance (PD) at home with a millimeter ruler and a mirror. The whole process takes about two minutes, and most adults end up with a number somewhere between 50 mm and 70 mm, with 63 mm being the average. Getting this number right matters because it tells a lens maker exactly where to place the optical center of each lens so it lines up with your eyes.
The Mirror and Ruler Method
This is the most widely recommended approach, including by the U.S. Department of Veterans Affairs. You need a standard ruler marked in millimeters and a well-lit mirror.
- Step 1: Stand about 8 inches from a mirror.
- Step 2: Hold the ruler flat against your brow, just above your eyes.
- Step 3: Close your right eye. Align the ruler’s 0 mm mark with the center of your left pupil.
- Step 4: Keep the ruler steady. Now close your left eye and open your right eye.
- Step 5: Read the millimeter mark that lines up with the center of your right pupil. That number is your PD.
The reason you close one eye at a time is to prevent parallax error. If both eyes are open while you try to read the ruler, your gaze shifts slightly and throws off the measurement. Repeat the process two or three times and average your results. If your readings vary by more than a millimeter or two, try again with better lighting or a steadier hand.
The Friend Method (Best for VR Headsets)
If you’re measuring PD for a VR headset rather than glasses, you want what’s called a “distance PD,” measured while your eyes focus on something far away. A friend makes this easier because you can look straight ahead naturally instead of staring into a mirror at close range.
Have your friend sit or crouch below your line of sight. Look at an object at least 10 to 20 feet away, keeping both eyes open. Your friend places the ruler’s zero mark at the center of one pupil and reads the millimeter mark at the center of the other. The key here: don’t look at the person measuring you, because that forces your eyes to converge inward and shrinks the reading. Take two or three measurements to confirm consistency.
Single PD vs. Dual PD
Most people only need a single number, their binocular PD, which is the total distance from one pupil center to the other. That’s what the methods above give you.
Some online eyewear retailers ask for a dual PD (also called monocular PD). This is two separate numbers representing the distance from each pupil to the center of your nose bridge. It’s typically written as something like 32/30, where the first number is the right eye and the second is the left. Most faces aren’t perfectly symmetrical, so these two numbers often differ by a millimeter or two.
To get your dual PD, use the same mirror method but measure from the center of each pupil to the middle of your nose bridge separately. If an online retailer gives you a single input field, enter your binocular PD. If it gives you two fields (right and left), enter your monocular measurements.
Smartphone Apps as an Alternative
Several free apps claim to measure PD using your phone’s camera. A study published in Cureus tested three of these apps against a professional-grade digital pupilometer in 44 subjects. The Warby Parker and Eye Measure apps performed best, with an average error of about 0.5 mm. A third app, PDCheck AR, had a higher average error of roughly 1.4 mm. The Warby Parker app most frequently landed within half a millimeter of the professional measurement.
If you don’t have a millimeter ruler handy, a well-designed app can get you close enough. But for strong prescriptions, even small PD errors become more noticeable, so the ruler method or a professional measurement is worth the extra effort.
What Typical Results Look Like
Adults generally fall between 50 mm and 70 mm, though a small percentage of people measure as low as 45 mm or as high as 80 mm. Your PD stabilizes once you’re done growing, so an adult measurement taken at 25 will still be accurate at 45. Children’s PD changes as their skull develops, which is one reason pediatric prescriptions need updating more frequently.
If your number seems unusually high or low, measure again before assuming something is off. A ruler that slipped, a glance in the wrong direction, or poor lighting can easily shift your reading by a few millimeters.
Why Getting It Wrong Causes Problems
When the optical centers of your lenses don’t align with your pupils, the lenses act like weak prisms, bending light in a direction your eyes don’t expect. The stronger your prescription, the more pronounced this effect becomes.
Horizontal misalignment (PD too wide or too narrow) often produces a “swimming” sensation, where objects seem to shift as you move your head. Vertical misalignment, which can happen with poorly fitted frames, tends to be harder to tolerate. People describe the floor looking concave or convex, or a persistent feeling of walking uphill or downhill. One telltale sign of prism-related problems: your vision feels clearer when you close one eye. That happens because the mismatch between what each eye sees disappears when only one eye is doing the work.
For mild prescriptions, being off by a millimeter or two rarely causes noticeable symptoms. For prescriptions above about +/- 4.00 diopters, even a 2 mm error can produce enough unwanted prism to cause eyestrain and headaches over the course of a day.