Pupillary distance (PD) is the distance in millimeters between the centers of your two pupils, and you can measure it at home with a millimeter ruler and a mirror. Most adults have a PD between 54 and 74 mm. Getting this number right matters because it tells the lens maker where to place the optical center of each lens so it lines up with your eye, giving you the clearest possible vision through your glasses.
Why PD Matters for Your Glasses
Every prescription lens bends light to correct your vision, but that correction works best through one specific point on the lens called the optical center. Your PD tells the lab exactly where to position that point. If the optical centers are even slightly off from your actual pupil positions, you end up looking through a part of the lens that introduces unwanted prismatic effect, which can cause eyestrain, headaches, or blurry vision.
How much a PD error matters depends on your prescription strength. If you have a mild prescription (say, -0.50 in both eyes), being off by a millimeter probably won’t bother you. But at -5.00 or higher, even a 1 mm error can cause significant eyestrain. The same goes for progressive lenses, which pack distance, intermediate, and near zones into a single lens. The corridor between those zones is narrow, so precise alignment is essential.
How to Measure PD With a Ruler and Mirror
The U.S. Department of Veterans Affairs recommends this straightforward method:
- Stand about 8 inches from a mirror. Get close enough to see your pupils clearly.
- Hold a millimeter ruler against your brow. Rest it flat across the bridge of your nose, just above your eyes, with the markings facing outward so you can read them in the mirror.
- Close your right eye. Align the ruler’s 0 mm mark with the center of your left pupil.
- Without moving the ruler, close your left eye and open your right eye. Look straight ahead.
- Read the millimeter mark that lines up with the center of your right pupil. That number is your binocular PD.
Repeat this two or three times and compare your results. If you’re getting the same number (or numbers within 1 mm of each other), you have a reliable measurement. Most people land somewhere between 57 and 65 mm.
Binocular PD vs. Monocular PD
The method above gives you a single binocular PD, which is the total distance from one pupil to the other. That’s all you need for standard single-vision glasses. But some orders, especially for progressive or bifocal lenses, ask for monocular PD: the distance from each pupil to the center of your nose bridge, listed as two separate numbers.
Most people’s faces aren’t perfectly symmetrical, so your left and right monocular PDs won’t be identical. You might be 31 mm on the right and 32 mm on the left, for example. The two monocular values should always add up to your binocular PD (in this case, 63 mm). When you see a PD written as “32/31” on a prescription or an online order form, the first number is typically the right eye and the second is the left.
How to Measure Monocular PD
Use the same ruler-and-mirror setup, but this time you’re measuring from the center of each pupil to the center of your nose bridge. Mark where the bridge of your nose sits on the ruler, then note the distance to each pupil separately. Alternatively, have a friend help: they can stand about arm’s length in front of you, hold the ruler across your brow, and read each measurement while you look straight ahead at a distant point behind them. Having someone else do it removes the challenge of closing one eye at a time, which can shift your gaze slightly.
Getting a More Accurate Measurement
The ruler method works well, but a few things can throw it off. Here’s how to minimize error:
- Look at a distant object. If you’re measuring with a friend’s help, stare at something across the room, not at their face. Looking at something close causes your eyes to converge inward, which shrinks your PD reading by a couple of millimeters.
- Keep the ruler level. Tilting it even slightly across your face changes the reading.
- Use good lighting. You need to see the center of your pupils clearly. Bright, even lighting makes your pupils smaller and easier to pinpoint.
- Measure multiple times. Three consistent readings give you confidence. If your results vary by more than 1 mm between attempts, try again with better lighting or a helper.
Smartphone apps that measure PD using your front camera also exist. Some use a reference object (like a credit card held to your forehead) to calibrate the scale. They can be convenient, but their accuracy varies by app and phone model. A careful ruler measurement is at least as reliable as most apps.
Is PD Included on Your Prescription?
Not always, and this catches many people off guard when they try to order glasses online. Under the FTC’s Eyeglass Rule, eye care providers must give you a copy of your prescription after an exam, but PD is not a federally required field on that prescription. Some states do require it. The FTC encourages providers to share PD measurements if they’ve taken them, noting that patients are likely entitled to a copy under federal or state medical records rules.
If your prescription doesn’t include PD, you have a few options. You can call your eye doctor’s office and ask for it, since they almost certainly measured it during your visit. You can measure it yourself using the methods above. Or you can visit an optical shop, where a technician will measure it for you, often for free if you’re purchasing frames there.
Near PD vs. Distance PD
The PD you measure while looking straight ahead at a distant point is your distance PD. When you focus on something close, like a book, your eyes angle inward and the effective distance between your pupils gets smaller. This is your near PD, and it’s typically about 3 to 4 mm less than your distance PD.
For reading glasses or the near portion of progressive lenses, some labs use a near PD. If an online retailer asks for it separately, you can subtract 3 mm from your distance PD as a reasonable estimate. But for most single-vision distance glasses, your standard (distance) PD is the only number you need.