Pupillary distance, the measurement between the centers of your pupils, is missing from most eye prescriptions because federal rules in the United States do not classify it as part of the prescription itself. The Federal Trade Commission’s Eyeglass Rule, which governs what eye doctors must release to patients, defines the prescription as the refractive correction your eyes need, and PD falls outside that definition. This regulatory distinction has real consequences: it can make ordering glasses online more complicated and has fueled an ongoing policy debate about whether the rules should change.
The Legal Line Between Prescription and Fitting
Your eyeglass prescription contains the optical values needed to correct your vision: sphere power, cylinder power, axis, and sometimes an add power for reading. These numbers describe what your eyes need regardless of which pair of frames you choose. PD, by contrast, describes where to position the optical centers of your lenses relative to your face. Regulators have historically treated the first category as medical data you have a right to and the second as a fitting measurement that belongs to whoever is making the glasses.
The FTC’s Ophthalmic Practice Rules, commonly called the Eyeglass Rule, require prescribers to give patients a copy of their prescription automatically after an eye exam. When the Commission updated these rules in 2024, it specifically sought public comment on whether the definition of “prescription” should be expanded to include pupillary distance. The agency noted that opticians, who are not authorized to examine eyes or determine prescriptions, may nonetheless conduct pupillary distance measurements as part of the fitting process.1Federal Register. Ophthalmic Practice Rules (Eyeglass Rule) In other words, the regulatory framework treats PD as something any trained eyewear professional can measure, not something only a doctor can determine.
A handful of U.S. states have passed their own laws requiring eye doctors to include PD on prescriptions or to provide it upon request. But most states follow the federal framework, which means your doctor is under no obligation to hand it over. Some offices will give it to you if you ask; others will not, or will charge a small fee for the measurement.
Why Some Eye Care Providers Prefer Not to Share It
The regulatory distinction is only part of the story. There is a straightforward business incentive at work. If you need your PD to order glasses online, and your doctor’s office is the only place that measured it, you are more likely to buy your glasses from that office’s optical shop. Online eyewear retailers have grown rapidly over the past decade, and brick-and-mortar practices that rely on frame and lens sales for a large share of their revenue have a financial reason to keep the fitting process in-house.
Providers who withhold PD often frame their reasoning in terms of patient safety. The argument goes that PD measurement should be done by a professional using proper equipment, and that an inaccurate measurement can lead to poorly made glasses that cause eyestrain or headaches. There is a kernel of truth here, especially for strong prescriptions and progressive lenses. But the argument conveniently overlaps with keeping the patient in the dispensary. For someone with a mild prescription ordering single-vision lenses, a small PD error is unlikely to cause problems. For someone with a high prescription or complex multifocal needs, the fitting process genuinely matters more.
What Happens When PD Is Wrong
Your PD tells the lab where to place the optical center of each lens. When the optical center lines up with your pupil, you are looking through the part of the lens that delivers the clearest, most distortion-free vision. When it does not, the lens effectively introduces a small amount of unwanted prism. For low prescriptions, this induced prism is trivial. For stronger prescriptions, even a couple of millimeters of misalignment can produce noticeable symptoms: eyestrain, headaches, or a sense that things look slightly “off” when you move your eyes.
The stakes rise with multifocal lenses. Progressive lenses pack distance, intermediate, and near zones into a single lens, and each zone occupies a narrow corridor. If the optical centers are misplaced horizontally because of an inaccurate PD, you might find yourself looking through the wrong zone or through a blended transition area that feels swimmy. A review in the ophthalmology literature emphasized that fitting multifocals involves many variables beyond PD alone, including segment height and frame angle, and that errors in any of them can cause dissatisfaction.2PubMed. Determining the vertical and horizontal positioning of multifocal and progressive lenses More recent work on free-form progressive lenses found that while PD measurements showed acceptable repeatability across different devices, other fitting parameters like fitting point height and pantoscopic angle were less repeatable and could introduce centration errors that affect vision or adaptation.3PubMed. Comparison of physiognomy and frame angle parameters using different devices to prescribe progressive addition lenses
So the concern about accuracy is legitimate for complex lens types. But for straightforward single-vision glasses in a moderate prescription, the margin of acceptable error is wider than many optical shops imply.
How PD Is Measured Professionally
In an eye care office, PD is most commonly measured with a pupillometer, a handheld device you look into while the clinician reads a digital display, or with a PD ruler held across the bridge of your nose while the clinician aligns reference marks with your pupils. Autorefractors, the machines that give a preliminary estimate of your prescription before the doctor refines it, also record PD as part of their measurement sequence.
A study comparing four methods side by side found that these professional tools generally agree well with each other, though not perfectly. Pupillometer and autorefractor readings for distance PD did not differ from each other, while a PD ruler tended to give slightly larger readings than a pupillometer. Mobile apps produced measurements that were shorter than the other three methods by a statistically meaningful margin.4PubMed Central. A Comparative Analysis of Interpupillary Distance Measurement Techniques Evaluation in Modern Times: From Rulers to Apps The practical takeaway is that professional instruments are reasonably interchangeable, while consumer-facing tools introduce more variability.
Measuring PD Yourself
If your prescription does not include PD and your provider will not supply it, you have a few options. The simplest is the mirror-and-ruler method: hold a millimeter ruler against your brow, close your right eye, align the zero mark with the center of your left pupil, then close your left eye and read the measurement at your right pupil. It is low-tech and imprecise, but with a few tries averaged together, most people can get within a couple of millimeters of a professional measurement.
Smartphone apps are more sophisticated and increasingly popular. Several online eyewear retailers offer their own apps, and third-party options exist as well. Research has evaluated a range of these apps with mixed results. One study found that the Eye Measure and Warby Parker apps achieved an average error of about half a millimeter compared to a pupillometer, while another app produced errors closer to 1.4 millimeters.5PubMed Central. Comparing the Effectiveness of Smartphone Applications in the Measurement of Interpupillary Distance A separate study found that one app called GlassifyMe agreed well with autorefractor measurements, with only about 0.2 mm mean difference.6Journal of Physics: Conference Series. Evaluation of Pupillary Distance (PD) Measurement using Smartphone-based Pupilometer The accuracy varies by app, and factors like lighting, head position, and how steady you hold the phone all matter.
For a standard single-vision prescription under about four or five diopters, a self-measured PD that is within a millimeter or two is generally fine. If your prescription is stronger, or if you are ordering progressive or bifocal lenses, a professional measurement is worth the effort.
Binocular Versus Monocular PD
When people talk about PD, they usually mean the total distance from the center of one pupil to the center of the other. This is binocular PD, and for most adults it falls somewhere in the range of 54 to 74 mm. But almost nobody’s face is perfectly symmetrical, which means the distance from the bridge of your nose to your right pupil may differ from the distance to your left pupil. These individual measurements are your monocular PDs, sometimes written as two numbers separated by a slash.
For single-vision lenses in a mild prescription, the difference between your two monocular PDs is often small enough to ignore. But in stronger prescriptions or progressive lenses, even a millimeter of asymmetry in lens positioning can shift the optical center away from where your eye actually sits. A study on progressive lens users found that among people with measurable facial asymmetry, a majority preferred lenses that were customized to account for that asymmetry over standard lenses with a symmetric inset.7Acta Ophthalmologica. Analysis of the relation between facial asymmetry and users’ preference between standard versus customized inset progressive addition lenses If you are ordering glasses online with a complex prescription, having monocular PD values rather than just a single binocular number gives the lab more to work with.
How to Ask for Your PD
The most direct approach is to ask your eye doctor or their staff during your exam. Many offices measure PD as a routine part of the visit even if they do not write it on the prescription form. If you ask before the exam starts, you are more likely to get it without friction. Phrasing it as “Could you include my PD on my prescription?” sometimes works better than asking after the fact, when the measurement may have already been recorded in the office’s internal system rather than on the document you take home.
If the office declines or charges a fee you would rather avoid, you can visit a different optical shop or optician and ask for a PD measurement. Many large retail chains with optical departments will measure PD at no charge, especially if you express interest in browsing their frames. Since PD measurement is a fitting procedure and not a medical exam, it does not require a licensed optometrist or ophthalmologist to perform.
Some online retailers will walk you through a video-call fitting or provide an app-based measurement tool as part of the ordering process. These are convenient but vary in quality, so it is worth comparing your self-measured result against at least one other method to catch obvious errors.
Why the Debate Keeps Coming Back
Consumer advocacy groups and online eyewear companies have pushed for years to have PD included in the standard prescription. Their argument is straightforward: if the point of the Eyeglass Rule is to let patients shop around for glasses, withholding a number they need to actually order those glasses undermines the rule’s purpose. The FTC has revisited this question multiple times, most recently in its 2024 rulemaking, where it again invited public comment on whether PD should be folded into the prescription definition.1Federal Register. Ophthalmic Practice Rules (Eyeglass Rule)
On the other side, professional associations representing optometrists argue that PD is a fitting parameter tied to a specific frame, not a fixed medical value that belongs on a prescription. They point out that PD can vary slightly depending on whether you are looking at something far away or up close, that it should ideally be measured with the actual frame you plan to wear, and that releasing a single number without context could lead to poorly fitted glasses. These points are technically valid but do not fully account for the fact that a binocular distance PD is stable enough for most single-vision orders, which represent the bulk of what people buy online.
The tension is unlikely to resolve cleanly because it sits at the intersection of consumer protection, professional scope of practice, and retail economics. Meanwhile, the practical workaround for most people is some combination of asking politely, measuring at home, and knowing when professional fitting genuinely matters.
PD Beyond Traditional Eyeglasses
Pupillary distance has become relevant in settings that have nothing to do with corrective lenses. Virtual reality and augmented reality headsets use interpupillary distance to align their optics with the wearer’s eyes. If a VR headset’s lenses are set too wide or too narrow for your face, the stereoscopic 3D effect breaks down, and you may experience blurred images, eye fatigue, or motion sickness. Most modern headsets include a physical or software-based IPD adjustment, and the recommended range typically spans the same adult values relevant to eyeglasses. Research has explored how mismatches between a headset’s optical spacing and the user’s actual PD can produce visual aftereffects even after removing the device, reinforcing that this is not just a comfort preference but a physiological concern.
Knowing your PD is useful for dialing in a VR headset, and since headset manufacturers cannot measure your eyes for you, they generally encourage users to find the number themselves. This is one more context where having PD on your prescription would save time, but it is also one more reason to measure it on your own if your doctor does not provide it. The measurement is the same regardless of whether you plan to use it for reading glasses or a gaming headset: center of one pupil to center of the other, looking straight ahead at a distant target.