Pupillary distance (PD) is the measurement, in millimeters, between the centers of your two pupils. It typically takes less than a minute to measure at home with a basic millimeter ruler and a mirror. The average adult PD falls between 54 and 68 mm, while children generally range from 43 to 58 mm.
This number matters because every prescription lens has an optical center, the point that should line up precisely with where your eye looks through the lens. If the optical center is off by even a couple of millimeters, you can experience eyestrain, blurry vision, or headaches. Getting your PD right is especially important when ordering glasses online, where no one is fitting the lenses to your face in person.
How to Measure PD With a Ruler and Mirror
You need a standard millimeter ruler (not inches) and a well-lit mirror. The method recommended by the U.S. Department of Veterans Affairs works reliably at home:
- Stand about 8 inches from the mirror.
- Hold the ruler horizontally against your brow, resting it just above your eyes.
- Close your right eye. Align the 0 mm mark with the center of your left pupil.
- Look straight ahead, then close your left eye and open your right eye.
- Read the millimeter mark that lines up with the center of your right pupil. That number is your binocular PD.
The key to this technique is the eye-switching step. By closing one eye at a time, you avoid the parallax error that would shift the ruler’s apparent position if both eyes were open and darting between the ruler and the mirror. Keep your head still and your gaze fixed straight ahead throughout.
Measuring With a Friend
Having someone else take the measurement is often more accurate than using a mirror, because they can look directly at your pupils without the awkward angle of reading a ruler in reverse. Ask the other person to stand at arm’s length, directly in front of you. You should focus on something behind them in the distance, like a doorframe or a picture on the wall. Focusing far away keeps your pupils centered and prevents them from converging inward.
Your helper then holds the ruler against your brow, aligns the 0 mm mark with the center of one pupil, and reads the millimeter line at the center of the other. They should close one of their own eyes while reading each pupil to eliminate parallax on their end, too.
Binocular PD vs. Monocular PD
Most online retailers ask for a single number, your binocular PD. This is the total distance from one pupil center to the other. Some sites and stronger prescriptions require monocular PD, which splits the measurement into two numbers: the distance from the bridge of your nose to each pupil individually. These two values often aren’t identical, since most faces are slightly asymmetric.
To get monocular PD, use the same ruler method but note where the center of your nose bridge falls on the ruler. Subtract that from each pupil’s position. You’ll end up with two numbers (for example, 31 mm left and 32 mm right) that add up to your binocular PD of 63 mm. If the retailer’s order form has two PD fields labeled “OD” and “OS,” OD is the right eye and OS is the left.
Common Mistakes to Avoid
The most frequent error is letting the ruler tilt. Even a slight angle skews the reading by a millimeter or two, which is enough to misalign the optical center of the lens. Keep the ruler flat and level across your brow the entire time.
Shifting your focus mid-measurement is another problem. If your eyes move to look at the ruler itself or wander around the mirror, your pupils shift position and the reading changes. Pick a fixed point, either your own reflection or a distant object if a friend is measuring, and hold your gaze there.
Take at least three separate readings and use the number that appears most often. Relying on a single measurement leaves room for a one-off error. If your three readings are 63, 64, and 63, your PD is 63 mm. If you get three different numbers each time, something in your technique is inconsistent, so slow down and try again.
Getting Your PD From Your Eye Doctor
Your optometrist or ophthalmologist can measure PD quickly during an eye exam, typically using a device called a pupillometer that gives a precise digital reading. However, PD is not always included on your prescription automatically. Under the FTC’s Eyeglass Rule, eye care providers must release your prescription after an exam, but the federal definition of a prescription covers the refractive specifications for your lenses, not necessarily PD. Some states do require PD to be included, while others leave it optional.
The FTC has encouraged providers to share PD measurements when they have them, noting that patients ordering glasses online will need the number. If your prescription doesn’t list a PD, you can call the office and ask for it. If they measured it during your visit, you’re likely entitled to that information under federal or state medical records rules. If they didn’t measure it, the at-home ruler method above will get you close enough for accurate lens placement.
What a Normal Range Looks Like
If your self-measurement lands between 54 and 68 mm and you’re an adult, you’re in the typical range. Children usually fall between 43 and 58 mm. A number outside these ranges isn’t necessarily wrong, since face shapes vary considerably, but it’s worth remeasuring to make sure. People with wider-set eyes can have PDs in the low 70s, and those with narrow features might be in the low 50s.
Your PD stays relatively stable once you reach adulthood. If you measured it a few years ago and your face hasn’t changed (no major dental surgery or facial trauma), the old number is probably still accurate. For children, PD should be remeasured with each new pair of glasses since their skulls are still growing.