Pupillary distance is not on your prescription because U.S. federal regulations do not classify it as part of the prescription itself. The Federal Trade Commission’s Eyeglass Rule, which governs what eye care providers must release to patients, defines a prescription in terms of lens power, cylinder, axis, and similar optical values. PD is treated as a fitting measurement, similar to frame size or the height at which a lens sits in front of your eye. That distinction has real consequences if you want to order glasses online, because you need that number and your doctor’s office has no legal obligation to hand it over.
The Regulatory Reason
The FTC’s Ophthalmic Practice Rules, commonly called the Eyeglass Rule, were updated in 2024 after a lengthy public comment period. During the rulemaking process, the Commission specifically asked whether the definition of “prescription” should be expanded to include pupillary distance. Commenters on both sides weighed in, with online retailers and consumer advocates pushing for inclusion, and many optometrists and ophthalmologists arguing against it. In the final rule, the FTC declined to add PD to the mandatory prescription contents.1Federal Register. Ophthalmic Practice Rules (Eyeglass Rule) – Section: VII. Miscellaneous Issues Raised in Comments / A. Pupillary Distance
The practical effect is straightforward: when your optometrist or ophthalmologist hands you a copy of your prescription after an exam, they are required to give you the sphere, cylinder, axis, add power (if applicable), and prism values. They are not required to measure your PD at all during the exam, and even if they do measure it, they are not required to write it on the prescription form. Some offices will include it as a courtesy, some will provide it if you ask, and some will refuse or charge a separate fee for the measurement. All of those responses are legal.
A handful of U.S. states have passed their own laws requiring PD on prescriptions, but this is the exception rather than the norm. If you live in a state without such a law, you are relying entirely on your provider’s willingness to share the number. This creates an obvious friction point for anyone trying to buy glasses somewhere other than the prescriber’s own optical shop.
Why Providers Push Back
If PD is just a number measured in millimeters, why would an eye doctor hesitate to include it? The answer is partly economic and partly clinical. On the economic side, selling glasses is a significant revenue stream for most optometry practices. Providing PD makes it easier for patients to order from online retailers or competing shops, which cuts into that revenue. Research from China found that separating eyeglass sales from optical care can actually lead to worse quality of eye care overall, because the economic incentives that fund thorough exams get disrupted when the glasses sale goes elsewhere.2PubMed Central. Effect of no eyeglasses sales on the quality of eye care: an experimental evidence from China That finding doesn’t justify withholding a measurement from a patient, but it does explain why the profession has been reluctant to hand over the keys to easy online ordering.
On the clinical side, providers argue that PD measurement is part of the dispensing process, not the diagnostic process. A prescription tells another qualified dispenser what optical correction your eyes need. PD tells the lab where to position the optical center of the lens relative to your pupil. In that framing, PD belongs to whoever is making the glasses, not to the prescription itself. Whether you find that argument persuasive depends on how you feel about the line between diagnosing a vision problem and solving it.
What PD Actually Does for Your Glasses
Your PD is the distance between the centers of your two pupils, measured in millimeters. For most adults, it falls somewhere between about 54 and 74 mm. When a lab makes your lenses, they position the optical center of each lens directly in front of the corresponding pupil. If the optical center is off to one side, you end up looking through a part of the lens that bends light at a slightly different angle than intended. That unintended bending is called a prismatic effect, and it can cause symptoms ranging from mild eyestrain to headaches and blurry vision.
A study that examined people wearing single-vision glasses found that roughly 57% of them were not looking through the optical center of their lenses. The average amount of misalignment was about 3.5 mm, and 40% of those with misaligned optical centers reported eyestrain or visual discomfort.3PubMed Central. Influence of prismatic effect due to decentration of optical center in ophthalmic lens That is a surprisingly high rate of error, and it underscores why getting PD right matters. The stronger your prescription, the more a millimeter or two of misalignment will affect you, because higher-powered lenses bend light more aggressively and amplify any offset.
For progressive lenses (no-line bifocals), accuracy matters even more. Progressive lenses have a narrow corridor of clear vision that transitions from distance at the top to near at the bottom. If the PD is off, the corridor shifts away from where your pupils naturally land, and you end up tilting your head or straining to find the sweet spot. Many people who complain that they “can’t get used to progressives” are actually dealing with lenses made to the wrong PD or fitting height.
How to Measure Your Own PD
If your provider won’t give you your PD, you have a few options. The most common at-home method uses a millimeter ruler and a mirror. You hold the ruler against your brow, close your right eye, align the zero mark with the center of your left pupil, then open both eyes and read the millimeter mark that falls on the center of your right pupil. It sounds simple, but parallax errors creep in easily, and your accuracy depends on keeping the ruler level and your head still.
Professional tools do this more reliably. A digital pupillometer, the handheld device your optician holds up to your face, is considered the gold standard. Autorefractors, the machines you look into during an eye exam, also record PD as part of their measurements. Research comparing these methods found that pupillometer readings and autorefractor readings generally agree with each other, while manual ruler measurements tend to read slightly larger, especially for near PD.4PubMed Central. A Comparative Analysis of Interpupillary Distance Measurement Techniques Evaluation in Modern Times: From Rulers to Apps A separate study found that manual and automatic measurements differed statistically but the difference was less than 1 mm in almost all conditions, which is generally considered within clinical tolerance for most prescriptions.5Journal of Optometry. Agreement and inter-session repeatability of manual and automatic interpupillary distance measurements
So a careful ruler measurement can get you close enough for single-vision lenses with a moderate prescription. For strong prescriptions or progressives, a millimeter of error matters more, and you may want a more precise method.
Are Smartphone PD Apps Any Good
Several smartphone apps now promise to measure your PD using your phone’s camera. The idea is appealing: skip the ruler, skip the office visit, and let technology handle it. But the accuracy varies considerably depending on which app you use.
One study compared three popular apps against a digital pupillometer. The Warby Parker app performed best, with the highest percentage of measurements falling within 1 mm of the pupillometer reading. The Eye Measure app performed similarly. The PDCheck AR app, however, was significantly less accurate than both.6PubMed Central. Comparing the Effectiveness of Smartphone Applications in the Measurement of Interpupillary Distance A different study found that an app called GlassifyMe produced measurements with no statistically significant difference from an autorefractor, with an average discrepancy of just 0.2 mm.7Journal of Physics: Conference Series. Evaluation of Pupillary Distance (PD) Measurement using Smartphone-based Pupilometer
The broader comparison research found that mobile app measurements as a category tended to read shorter than pupillometer or autorefractor readings.4PubMed Central. A Comparative Analysis of Interpupillary Distance Measurement Techniques Evaluation in Modern Times: From Rulers to Apps That consistent bias is worth knowing about. If you use an app and get a number, it may be a millimeter or so smaller than what a pupillometer would give. For a low-to-moderate prescription in single-vision lenses, that small difference is unlikely to cause problems. For a strong prescription or progressives, it could push you past the threshold where you start noticing eyestrain or distortion.
The best strategy if you are relying on an app is to take multiple measurements across different sessions, toss out any obvious outliers, and average the rest. Lighting matters: apps that use the front-facing camera need your pupils to be clearly visible, which means even, non-glaring light and a neutral background. If you wear contacts during the measurement, take them out first, since some contacts can subtly shift the apparent position of your pupil.
Binocular PD vs. Monocular PD
When you see a single PD number like “64 mm,” that is your binocular PD, the total distance from one pupil center to the other. But most people’s faces are not perfectly symmetrical. Your nose may be slightly off-center, or one eye may sit a fraction of a millimeter further from the bridge than the other. That means the distance from your right pupil to the center of your nose bridge is not necessarily identical to the distance from your left pupil to the bridge.
Monocular PD splits the measurement into two numbers: one for each eye measured from the center of the bridge of your nose. You might see it written as “31/33” instead of “64,” meaning 31 mm for one eye and 33 mm for the other. For single-vision lenses at low to moderate power, binocular PD is usually good enough. But for strong prescriptions and especially for progressives, monocular PD gives the lab more precise positioning information. If you are ordering progressives online, look for a retailer that accepts monocular PD, and try to get that split measurement rather than a single number.
What Online Retailers Actually Do With Your PD
When you order glasses online, you typically enter your PD yourself during checkout. The retailer’s lab uses that number to position the optical center of each lens. Nobody double-checks your measurement against a clinical standard. This is one of the concerns raised about online spectacle ordering: the quality of the finished product depends on the accuracy of a measurement the consumer provides without professional oversight.8PubMed. Accuracy and Repeatability of Internet-ordered Spectacle Lenses
If you enter a PD that is off by 2 or 3 mm, the resulting prismatic effect may be barely noticeable with a weak prescription, or it may give you persistent headaches with a strong one. Most online retailers offer a return window, and some have started building virtual try-on tools that double as PD measurement features. But the responsibility for accuracy still falls on you. There is no technician verifying the measurement before the lenses get cut.
Brick-and-mortar optical shops measure your PD as part of the fitting process, which is one reason providers frame PD as a dispensing measurement rather than a prescription element. In a traditional workflow, the prescriber writes the lens power, and the dispenser handles everything about how that power gets delivered to your eye: frame selection, PD, fitting height, vertex distance, and lens material. The tension with online ordering is that this workflow assumes the prescriber and dispenser are in separate professional roles, while the patient now wants to be their own dispenser.
When a Wrong PD Causes Real Problems
For most people with a mild prescription, a PD error of a couple of millimeters will not produce dramatic symptoms. You might notice mild fatigue after extended reading, or a vague sense that something feels “off” without being able to pinpoint it. But as prescription strength increases, the consequences scale up. The induced prismatic effect from a decentered optical center follows a predictable relationship with lens power: higher power means more prism per millimeter of offset.
The study that found 57% of single-vision wearers had misaligned optical centers also found that the induced prismatic effects ranged from about -0.7 to +0.6 prism diopters in that sample.3PubMed Central. Influence of prismatic effect due to decentration of optical center in ophthalmic lens That range is relatively small, and many people adapt to it without realizing anything is wrong. But for someone with a strong prescription, the same millimeter offset produces a larger prismatic effect, and adaptation becomes harder. Symptoms can include double vision at certain distances, difficulty with depth perception, and headaches that worsen over the course of the day.
Children are a particular concern. A child’s PD is smaller and changes as they grow, and kids may not articulate that their glasses feel wrong. If a child’s glasses are made with an adult’s PD assumptions or an outdated measurement, the resulting prismatic mismatch can interfere with visual development. Pediatric prescriptions should always be dispensed with a freshly measured PD, ideally using a pupillometer rather than a ruler, since children are less likely to hold still for a manual measurement.
PD Changes Over Your Lifetime
Your PD is not entirely fixed. It increases during childhood and adolescence as your skull grows, typically stabilizing somewhere in your late teens or early twenties. After that, it stays remarkably stable for decades. Small changes in the range of a millimeter can occur with aging, partly due to changes in facial soft tissue, but these shifts are rarely large enough to matter for lens fitting.
What does change with age is your near PD. When you look at something close, your eyes converge inward, and the effective distance between your pupils shrinks. Near PD is typically 3 to 4 mm less than distance PD. This distinction matters if you wear reading glasses or bifocals, because the optical centers for the near-vision portion of the lens need to be positioned slightly closer together than the distance portion. Some prescriptions and some online order forms ask for both distance PD and near PD. If you only have one number, it is almost certainly your distance PD, and a near PD can be estimated by subtracting 3 mm, though a direct measurement is more accurate.
The convergence amount also varies by individual. People with a wider PD tend to converge more in absolute terms when focusing up close, but the 3-to-4 mm rule of thumb holds for most adults. If you have ever noticed that reading glasses feel fine for a book held at arm’s length but uncomfortable when you hold your phone closer, part of the issue may be that the near PD built into the lenses does not match your actual convergence at that shorter distance.
Getting Your PD From an Old Pair of Glasses
If you already own a pair of glasses that feel comfortable and give you clear vision, your PD is effectively encoded in those lenses. An optician can measure the distance between the optical centers of your current lenses using a device called a lensometer or focimeter. That gives your PD as it was used for that pair. Some online guides suggest holding your glasses up to a ruler and measuring between the center of each lens, but this is imprecise because the geometric center of the lens is not necessarily the optical center, especially if the lenses were intentionally decentered during manufacturing.
If you visit an optical shop and ask them to read your lenses on a lensometer, most will do it quickly. Some may charge a small fee. The measurement gives you not only your PD but also a verification of your prescription, which can be useful if you have lost your paper prescription and your exam has not yet expired. Keep in mind that if the old glasses were made with an inaccurate PD in the first place, you will just be replicating the same error. If those glasses have always felt a little off, start fresh with a proper measurement.