Your pupillary distance (PD) is the space between the centers of your two pupils, measured in millimeters. Most adults fall between 58 and 68 mm, with an average of about 62 mm for women and 64 mm for men. You need this number to order glasses online, and there are several reliable ways to get it at home.
What PD Is and Why It Matters
When lenses are made for your glasses, the optical center of each lens needs to line up with your pupils. If the PD used to make your glasses is off, you’ll be looking through the wrong part of the lens. That can cause eyestrain, headaches, and blurry vision, especially with stronger prescriptions.
There are two types of PD measurements. Binocular PD is the total distance from one pupil center to the other. Monocular PD (sometimes called “dual PD”) measures each eye separately: the distance from the center of each pupil to the bridge of your nose. Most faces aren’t perfectly symmetrical, so the two numbers won’t be identical. If you’re ordering progressive lenses or bifocals, monocular PD is important for getting the lens zones positioned correctly.
The Mirror and Ruler Method
This is the most common DIY approach, and it works well with a little patience. You’ll need a millimeter ruler (not inches) and a well-lit mirror. The U.S. Department of Veterans Affairs recommends these steps:
- Stand about 8 inches from a mirror.
- Hold a millimeter ruler against your brow, flat across your face.
- Close your right eye. Align the ruler’s 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 closing-one-eye-at-a-time part is essential. If both eyes are open, you’ll shift your gaze slightly toward the mirror and throw the measurement off. Repeat three or four times and take the average. If your readings vary by more than a millimeter, try again with better lighting or ask someone to help.
To get monocular PD, have a friend stand in front of you at arm’s length. While you look straight ahead at a distant point, they measure from the center of each pupil to the center of your nose bridge. You’ll end up with two numbers, like 31.5/33, representing your right and left eye separately.
Using a Smartphone App
Several free apps claim to measure PD using your phone’s camera. Their accuracy varies significantly. A study comparing three popular apps against professional equipment found that the best-performing app (GlassifyMe) differed from the clinical measurement by only 0.2 mm on average, which is close enough for most glasses orders. Other apps in the same study were off by 0.6 mm and 2.1 mm, which starts to matter for stronger prescriptions.
If you go the app route, look for one that asks you to hold a reference object (like a credit card) near your face. The card gives the software a known size to calibrate against, which improves accuracy. Warby Parker’s online tool, for example, asks you to place a standard card under your nose and take a photo. Make sure the card is touching your face and you’re looking straight at the camera.
Getting Your PD From an Eye Doctor
Your eye doctor or optician measures your PD during a glasses fitting, but that number doesn’t always show up on your prescription. The FTC’s Eyeglass Rule requires prescribers to release your prescription after an exam, but PD isn’t universally included. Some states do require it. If your PD was measured during your visit, the FTC encourages providers to hand it over, and you’re likely entitled to it under federal or state medical records rules. It doesn’t hurt to simply ask at your next appointment.
If you already have a pair of well-fitting glasses, an optician can sometimes reverse-engineer your PD from the lens markings, or you can bring the glasses to a local optical shop and ask them to measure it for you.
Adjusting PD for Reading Glasses
When you focus on something close, your eyes angle inward slightly, which effectively shortens the distance between your pupils. If you’re ordering dedicated reading glasses, you need a “near PD” rather than the standard distance PD.
The calculation is simple: subtract 3 mm from your binocular PD. A distance PD of 63 mm becomes a near PD of 60 mm. If you’re using monocular measurements, subtract 1.5 mm from each eye. So a dual PD of 33/31 becomes 31.5/29.5 for reading glasses. Most online retailers will ask whether you need distance or near PD, and now you know which to enter.
Tips for an Accurate Measurement
Whichever method you choose, a few details make a real difference. Measure in good, even lighting so your pupils are a normal size. Dilated pupils in dim light are harder to center on, and constricted pupils in bright light can be too small to read precisely. Natural daylight from a window works well.
Always look straight ahead, not down at the ruler or up at the mirror’s edge. Your pupils shift position when your gaze moves, even slightly. If you’re having a friend help, ask them to close one eye while reading each measurement to avoid parallax error (seeing the ruler at an angle).
Take at least three measurements and compare them. If they’re all within 1 mm of each other, you’re in good shape. If your result falls outside the 54 to 74 mm range, something likely went wrong, so start over. And if you wear a strong prescription (above roughly +/- 4.00), consider getting a professional measurement. The higher your prescription, the more sensitive your vision is to even a 1 mm PD error.