Pupillary distance (PD) is the space in millimeters between the centers of your two pupils. You can measure it at home with a millimeter ruler and a mirror in about 30 seconds, and for most people ordering glasses online, that’s accurate enough. The average adult PD falls around 63 mm, with most adults landing somewhere between 50 mm and 70 mm.
The Ruler and Mirror Method
This is the most common DIY approach, and it’s the method recommended by the U.S. Department of Veterans Affairs. Here’s how it works:
- Stand about 8 inches from a mirror.
- Hold a millimeter ruler against your brow, flat across the bridge of your nose.
- Close your right eye and align the ruler’s 0 mm mark with the center of your left pupil.
- While keeping the ruler still, close your left eye and open your right eye.
- The millimeter mark that lines up with the center of your right pupil is your PD.
The eye-switching step is important. If both eyes are open while you try to read the ruler, your focus shifts and throws off the alignment. Closing one eye at a time keeps each measurement anchored to the correct pupil. Run through the process three or four times and average your results. A study published in Investigative Ophthalmology & Visual Science found that manual ruler measurements had a repeatability of about ±0.75 mm, meaning your readings may vary by close to a millimeter each time.
Using a Friend Instead of a Mirror
If the mirror method feels awkward, have someone else take the measurement. Stand facing each other at arm’s length. You look at something in the distance (not at the person measuring) so your eyes are relaxed and pointed straight ahead. The other person holds the ruler across your brow and reads the distance from one pupil center to the other. This avoids the alternating-eye technique entirely, since the measurer can see both of your pupils at once.
Smartphone Apps and Online Tools
Several apps and websites now measure PD using your phone’s camera. Some ask you to hold a standard-sized object near your face, like a credit card or student ID, as a reference scale so the software can calculate real-world distances from the image. Others use the phone’s front-facing camera and on-screen guides to map your pupil positions directly.
These tools are surprisingly consistent. The same study that tested ruler accuracy compared two smartphone apps (Dotty and Veero) against both manual rulers and portable pupillometers. The apps showed repeatability of ±0.22 to ±0.28 mm, considerably tighter than the ±0.75 mm from a ruler. The tradeoff is a small but consistent bias: smartphone measurements tended to read about 0.7 mm higher than ruler measurements. For standard single-vision glasses, that difference is unlikely to cause problems. For progressive lenses or strong prescriptions, it could matter.
Binocular PD vs. Monocular PD
The number you get from the ruler method is your binocular PD: the total distance from one pupil to the other. That’s the single number most online glasses retailers ask for, and it’s all you need for basic single-vision lenses.
Monocular PD splits that measurement into two numbers, one for each eye, measured from the center of each pupil to the bridge of your nose. Most people’s faces aren’t perfectly symmetrical, so your right and left monocular PDs may differ by a millimeter or two. Progressive lenses (which have different focusing zones for distance, intermediate, and near vision) require monocular PD for a proper fitting, because the optical center of each lens needs to align precisely with each individual eye. If your prescription calls for progressives, your eye care provider will typically measure monocular PD for you.
Distance PD vs. Near PD
Your eyes angle slightly inward when you focus on something close, like a book or a phone screen. That convergence makes the effective distance between your pupils a bit smaller at reading distance than when you’re looking across the room. If you’re ordering dedicated reading glasses, subtract 3 mm from your binocular PD to get your near PD. For monocular measurements, subtract 1.5 mm from each eye’s number.
Standard distance glasses and most everyday prescriptions use the distance PD. You only need the near PD if you’re getting single-vision readers specifically for close-up work.
What Happens if Your PD Is Wrong
When lenses are made, the optical center of each lens is positioned to sit directly in front of your pupil. If your PD measurement is off, those optical centers shift away from your line of sight, and the lens bends light at an unintended angle. Opticians call this “induced prism.” The stronger your prescription, the more a PD error matters, because the amount of unwanted prism scales with both the lens power and the distance the center is off.
A PD that’s a couple of millimeters off in a mild prescription might cause no noticeable issues. But in stronger prescriptions, even 2 to 3 mm of error can produce headaches, eye strain, blurred vision, nausea, or in extreme cases double vision. If a new pair of glasses feels “off” after a reasonable adjustment period, an incorrect PD is one of the most common culprits.
How Professionals Measure PD
At an eye care office, your PD is usually measured with a device called a pupillometer. You look into a small handheld instrument while it illuminates your eyes with infrared light and captures a rapid series of images. The whole process takes a few seconds and is accurate to fractions of a millimeter. Some offices still use a manual technique similar to the friend method described above, but with a specialized ruler called a PD stick and a penlight to create a tiny reflection on each cornea for a more precise reference point.
Your PD stays stable once you’ve finished growing, so a measurement taken during one eye exam remains valid for future glasses orders. If you’ve had your PD measured professionally before, you can request it from your provider’s records rather than remeasuring at home. Some practices include it on the prescription itself, though they aren’t always required to.
Tips for a More Accurate Home Measurement
Good lighting makes a significant difference. Your pupils constrict in bright light, becoming smaller and easier to pinpoint. Measure in a well-lit room, or position a lamp so it illuminates your face without creating glare on the mirror. Make sure the ruler sits level across your brow and doesn’t tilt, since even a slight angle introduces error over the 50 to 70 mm span you’re measuring. Take at least three readings and check that they cluster within 1 mm of each other. If your numbers are bouncing around by 2 mm or more, the ruler is probably shifting between attempts.
For children, the mirror method rarely works well because it requires holding still and switching eyes on command. Having another adult take the measurement is far easier. Children’s PDs are smaller, typically in the 40 to 55 mm range, and will continue to change as the child grows, so remeasuring before each new pair of glasses is worth doing.