Pupillary distance is frequently absent from eyeglass prescriptions, and that missing number is the single most common reason people get stuck when trying to order glasses online. In the United States, the federal rule requiring prescribers to hand over your prescription after an exam does not specifically mandate that PD be included. Whether your PD appears on that slip of paper depends on your state’s laws, your prescriber’s habits, and whether you ask for it. Understanding why it goes missing, how to get it, and why it matters for your lenses will save you frustration and possibly a headache from poorly made glasses.
Why PD Is Often Missing
The federal Eyeglass Rule, enforced by the FTC, requires optometrists and ophthalmologists to give you a copy of your eyeglass prescription immediately after your exam at no extra charge. But the rule defines a prescription as “the written specifications for lenses for eyeglasses which are derived from an eye examination, including all of the information specified by State law.”1Federal Register. Ophthalmic Practice Rules (Eyeglass Rule) That language is important: sphere, cylinder, axis, and add power are universally considered part of the prescription. PD, on the other hand, is treated differently in many states because it is considered a fitting measurement rather than a refractive finding. Unless your state specifically requires PD on the prescription form, your eye doctor can legally leave it off.
This distinction is not arbitrary from the prescriber’s perspective. The power of your lenses is determined by your eye’s optics, but PD is a measurement of your face. Traditionally, the optical shop or lab measured PD when you chose your frames, because the frame you select can influence exactly where the optical centers of the lenses need to sit. Some practitioners argue PD belongs to the dispensing step, not the examining step. Others, including many consumer advocates, counter that withholding PD effectively forces patients to buy glasses from the prescriber’s own shop. The practical upshot for you: if PD is not on the paper you were handed, you are not alone, and it does not mean your exam was incomplete.
How to Get Your PD if It Is Not Listed
The simplest route is to ask. Many offices will measure PD and write it down if you request it during your appointment. If you have already left, call the office. Some will provide it over the phone; others will ask you to come back. If you have purchased glasses from that office before, your PD is almost certainly in their records and a staff member can look it up. A few states have enacted laws requiring PD on every prescription, so check your state’s optometric board website if you want to know your rights specifically.
If the office will not release it, or if you would rather avoid the call, you can measure PD yourself or use a digital tool. Both approaches work, but each comes with caveats covered below.
What PD Actually Does for Your Lenses
PD tells the lab where to position the optical center of each lens relative to your pupils. When those centers line up with your line of sight, light passes through the thinnest, least-distorting part of the lens and you see clearly without unnecessary strain. When they do not line up, the lens bends light at a slight angle, creating what opticians call a prismatic effect. Your eyes then have to work harder to compensate, which can cause eyestrain, headaches, and blurred vision.
A study measuring single-vision glasses found that roughly 57% of wearers were not looking through the true optical center of their lenses, experiencing small prismatic shifts. Among those with misaligned optical centers, about 40% reported symptoms of visual discomfort.2PubMed Central. Influence of prismatic effect due to decentration of optical center in ophthalmic lens The average decentration in that study was 3.5 mm, which sounds tiny but is enough to matter for moderate-to-strong prescriptions. The stronger your prescription, the more a millimeter of misalignment translates into unwanted prism.
Binocular PD vs Monocular PD
Your prescription might list PD as a single number (say, 63 mm) or as two numbers (31/32). The single number is your binocular PD, the total distance from the center of one pupil to the center of the other. The two-number version gives your monocular PD for each eye, measured from the center of the bridge of your nose to each pupil separately.
Monocular PD matters because almost nobody’s face is perfectly symmetrical. A large study of over 1,200 patients found that 79% had some degree of nasopupillary asymmetry, meaning the distance from the nose bridge to the right pupil was not the same as the distance to the left. In that sample, roughly 73% had the right eye farther from center than the left.3PubMed Central. Nasopupillary Asymmetry The average asymmetry was about 1.3 mm, and while most cases were mild, some reached as high as 6 mm. For low prescriptions, a binocular PD is usually fine. For anything moderate or strong, or for progressive lenses, monocular values give the lab a much better shot at centering each lens properly.
Distance PD vs Near PD
When you look at something far away, your eyes are essentially parallel and your pupils are at their widest separation. When you focus on something close, your eyes converge inward and the effective distance between your pupils shrinks. This means your PD for distance viewing is always a few millimeters larger than your PD for reading.
Research measuring both in adult males found that the near PD was roughly 4 to 5 mm less than the distance PD, with the gap widening slightly for people with wider-set eyes.4PubMed Central. Influence of age on measured anatomical and physiological interpupillary distance (far and near), and near heterophoria, in Arab males For someone with a far PD of 63 mm, the near PD might be around 59 mm. If your prescription is for single-vision distance glasses, the distance PD is the one you need. If it is for reading glasses, you need the near PD. For progressive or bifocal lenses, the lab ideally has both, because different zones of the lens serve different viewing distances.
Most prescriptions that do include PD list only the distance value. If you order reading glasses online with a distance PD and no adjustment, the optical centers will be set too far apart for close-up work. Some online retailers automatically subtract a standard amount (usually 3 mm) when you order readers, but not all do, and a standard subtraction ignores individual variation. If you are ordering dedicated reading glasses, confirm which PD is being used.
When PD Accuracy Matters Most
Not every pair of glasses demands sub-millimeter PD precision. For a mild prescription in single-vision lenses, being off by a millimeter or two in PD is unlikely to cause noticeable symptoms. The stakes go up sharply in two situations: strong prescriptions and progressive lenses.
With stronger lenses, even a small decentration produces a larger prismatic effect. A study of over-the-counter reading glasses found that optical centers were vertically displaced by 3 mm or more in 11% of the pairs tested, and PD fell outside the standard range of 60 to 70 mm in 5% of them.5PubMed Central. Displacement of optical centers in over-the-counter readers: a potential cause of diplopia For weak magnification, most people tolerate that mismatch. For prescription lenses with significant power, the same displacement can cause double vision or persistent eye fatigue.
Progressive lenses are even more demanding. These lenses have a gradient of power from top to bottom, with a narrow corridor of clear vision in between. If the optical centers are not positioned to match your pupils precisely, you can end up peering through the wrong part of the lens at every distance. Accurate PD, along with fitting height and frame angle measurements, is considered essential for free-form progressive lens prescriptions.6PubMed Central. Comparison of physiognomy and frame angle parameters using different devices to prescribe progressive addition lenses If you wear progressives, getting PD right is not optional.
Measuring PD Yourself
If you cannot get PD from your prescriber, you have two main self-measurement options: a ruler and a mirror, or a smartphone app.
The ruler method is straightforward. Stand about arm’s length from a mirror, hold a millimeter ruler across the bridge of your nose, close your right eye, and align the ruler’s zero with the center of your left pupil. Then, without moving the ruler, close your left eye and open your right, and read the millimeter mark that lines up with your right pupil’s center. That number is your binocular distance PD. The technique is simple but has a real weakness: parallax. Because you are using one eye at a time, small head movements can shift your reading by a millimeter or more, and research consistently shows that PD ruler measurements tend to run slightly larger than those from a digital pupillometer.7PubMed Central. A Comparative Analysis of Interpupillary Distance Measurement Techniques Evaluation in Modern Times: From Rulers to Apps Repeating the measurement several times and averaging can help.
Smartphone apps use your phone’s camera (sometimes with a reference card like a credit card held up for scale) to estimate pupil positions. The accuracy varies by app. One comparative study found that Warby Parker’s app had the highest percentage of measurements falling within 1 mm of a digital pupillometer reading, outperforming PDCheck AR and Eye Measure.8PubMed Central. Comparing the Effectiveness of Smartphone Applications in the Measurement of Interpupillary Distance Another study found that the GlassifyMe app produced results with no statistically significant difference from an autorefractometer, with a mean discrepancy of just 0.2 mm.9Journal of Physics: Conference Series. Evaluation of Pupillary Distance (PD) Measurement using Smartphone-based Pupilometer
That said, the overall picture is mixed. The same study that compared rulers, pupillometers, autorefractors, and apps found that mobile apps tended to produce significantly smaller distance PD readings than other methods.7PubMed Central. A Comparative Analysis of Interpupillary Distance Measurement Techniques Evaluation in Modern Times: From Rulers to Apps A separate analysis concluded that while manual and automatic PD measurements were statistically different from each other, the differences were generally not large enough to be clinically meaningful.10PubMed Central. Agreement and inter-session repeatability of manual and automatic interpupillary distance measurements Translation: for a mild single-vision prescription, a decent app or a careful ruler measurement is usually good enough. For a strong prescription or progressives, a professional measurement with a pupillometer is worth the effort.
How PD Varies Across People
PD is not one-size-fits-all, and the range is wider than many people expect. A large meta-analysis found that pooled mean PD across different populations ranged from about 56 mm to nearly 70 mm, with West African populations averaging the widest and certain other groups averaging narrower.11PubMed Central. The Interpupillary Distance Differs Between Ethnicities and Associates with Horizontal Strabismus Patterns: Evidence from a Systematic Review and Meta-Analysis Men’s PD is typically 2 to 3 mm wider than women’s in the same population.12PubMed Central. The Interpupillary Distance Differs Between Ethnicities and Associates with Horizontal Strabismus Patterns: Evidence from a Systematic Review and Meta-Analysis This sex difference is consistent enough that “average adult PD” without specifying sex is a somewhat blurry number.
Age matters too. PD increases steeply through early childhood, then more gradually, and largely stabilizes by late adolescence. A study of over 1,300 subjects from infancy to age 19 modeled PD growth as a curve that rises quickly in the first few years, then levels off. Boys’ PD was on average about 1.6 mm wider than girls’ across the entire growth period.13PubMed. Normal values and standard deviations for pupil diameter and interpupillary distance in subjects aged 1 month to 19 years For children who need glasses, this means PD should be re-measured at each visit, because last year’s value may already be too small.
Why “One-Size” Reading Glasses Often Feel Wrong
Cheap reading glasses from the drugstore are ground to a single fixed PD, usually around 62 to 64 mm. If your PD is substantially wider or narrower, the optical centers will not match your eyes. The over-the-counter reader study cited earlier found that monocular PD was asymmetric by 5 mm or more in 4% of pairs, and the interpupillary distance of the glasses themselves fell outside the 60-to-70 mm range in 5% of the sample.5PubMed Central. Displacement of optical centers in over-the-counter readers: a potential cause of diplopia For occasional, low-power use, most people adapt to the slight mismatch without realizing it. For sustained reading at moderate magnification, the mismatch can produce headaches, eye fatigue, or the unsettling sensation that text is slightly doubled. If drugstore readers have always felt “off” to you despite seeming like the right power, PD mismatch is a likely culprit.
Pupil Center Is Not Always Where You Think It Is
One subtlety worth knowing: the geometric center of your pupil is not quite in the center of your cornea. Research has measured the pupil center as sitting about 0.34 mm toward the temple side of the corneal light reflex, which is the bright dot you see when a light shines at your eye.14PubMed. Pupil center shift relative to the coaxially sighted corneal light reflex under natural and pharmacologically dilated conditions This offset is small and consistent across different lighting conditions, so for standard eyeglass fitting it is already factored into the instruments opticians use. But it is one reason why truly precise measurements use dedicated pupillometers or digital centration devices rather than eyeballing things with a ruler. The instruments are calibrated to account for this natural offset; your bathroom mirror is not.
Your pupil also shifts slightly when it dilates or constricts. In bright light, your pupils are small and slightly more centered. In dim conditions, they open wider and shift a fraction of a millimeter. For everyday eyeglasses, this movement is too small to worry about. For specialty lenses, such as those used in refractive surgery planning, the distinction between pupil center under different lighting is taken more seriously.14PubMed. Pupil center shift relative to the coaxially sighted corneal light reflex under natural and pharmacologically dilated conditions
What to Do When Ordering Online
If you are ordering glasses from an online retailer and need a PD, here is a practical sequence. First, check your prescription slip and any paperwork from your last visit. PD sometimes appears on a separate fitting card or in the “additional measurements” section rather than on the prescription itself. If it is not there, call your eye doctor’s office and ask them to provide it. Many offices have it on file from when they fitted you previously. If that fails, use a smartphone app with decent accuracy (Warby Parker and GlassifyMe have both shown reasonable agreement with professional instruments in research). Measure at least three times and take the middle value. For single-vision lenses with a mild-to-moderate prescription, a self-measured PD within a millimeter of your true value is unlikely to cause problems.
If you wear progressives, bifocals, or have a prescription stronger than about plus or minus four diopters, consider getting your PD measured professionally. Some optical shops will measure PD for a small fee even if you are not buying glasses from them. The cost is usually minimal, and the peace of mind is worth it for lenses where precision directly affects how well you see through different zones.
One more detail that trips people up online: some order forms ask for binocular PD, some ask for monocular PD per eye, and some ask for both. If you only have a single binocular number, dividing it by two gives you a rough monocular estimate, but as the nasopupillary asymmetry data shows, the split is rarely perfectly even. When in doubt, enter the binocular value and let the lab’s default centering handle the rest; for most people’s level of asymmetry, this produces an acceptable result.