Which PD measurement you need depends entirely on what the glasses are for. Use your distance PD for single-vision distance glasses, your near PD for dedicated reading glasses, and your distance PD for progressive or bifocal lenses (the lab calculates the near portion from there). This sounds simple, but the difference between the two numbers matters more than most people realize, and getting it wrong can cause eyestrain, headaches, or blurred vision at the working distance you actually need.
Why There Are Two PD Numbers in the First Place
Your pupils are not fixed targets. When you look at something far away, your eyes aim roughly parallel, and the distance between the centers of your pupils is at its widest. When you shift focus to something close, like a book or a phone, both eyes rotate inward to converge on that near object. This convergence pulls the pupils closer together. The measurement taken while looking at a distant target is your distance PD (sometimes called far PD or FIPD), and the measurement taken while focusing on a near object is your near PD (NIPD).
The gap between the two is not trivial. A study measuring both values in adult males found that overall distance PD ranged from 56 to 73 mm while near PD ranged from 50 to 70 mm. For a person with a distance PD of 63 mm, the corresponding near PD was about 4.4 to 4.6 mm smaller, depending on the measurement method used.1Dove Press / Taylor & Francis (Clin Ophthalmol.). Influence of age on measured anatomical and physiological interpupillary distance (far and near), and near heterophoria, in Arab males That 4 to 5 mm difference is enough to shift the optical center of a lens away from where your eye is actually looking, which creates unwanted prismatic effects.
How the Wrong PD Creates Problems
Every prescription lens has an optical center, the point where light passes through without being bent sideways. Ideally, that center lines up exactly with your pupil for the distance at which you will be using the glasses. When it does not, the lens acts partly like a prism, bending light to one side. The further the optical center sits from where your eye is looking, the more unwanted prism you get. This relationship is described by a principle in optics called Prentice’s rule, which essentially says that prism increases with both the lens power and the distance of misalignment.2Wiley Online Library. Influence of prismatic effect due to decentration of optical center in ophthalmic lens
If you order reading glasses using your distance PD, the optical centers will be set too far apart for the convergence your eyes are performing at a near distance. Each eye ends up looking through the lens slightly off-center. With low prescriptions, the resulting prism is small enough that your eye muscles can compensate without much fuss. With stronger prescriptions, even a few millimeters of misalignment can produce noticeable eyestrain, double vision, or that uncomfortable “pulling” feeling after a few minutes of reading. A study analyzing ready-made readers found a statistically significant mismatch between the fixed optical-center distance of off-the-shelf glasses and the actual near PD of patients, with women experiencing a larger discrepancy than men.3CrossRef (African Vision and Eye Health). Analysis of readymade readers and near–inter-pupillary distance for presbyopic patients in optometric practice in Cape Town, South Africa
The practical takeaway is that stronger lenses are less forgiving of PD errors, and near-use glasses are where the mismatch is most likely to occur, because people often only know their distance PD.
Which PD for Which Glasses
The rule itself is straightforward once you know the principle behind it:
- Single-vision distance: Use your distance PD. These glasses are meant for driving, watching TV, or seeing across a room, all situations where your eyes are looking roughly straight ahead.
- Single-vision reading: Use your near PD. These are dedicated close-up glasses, so the optical centers need to match where your pupils actually sit when converged on a book or screen.
- Computer or intermediate glasses: Use a PD somewhere between your distance and near measurements. The exact value depends on your working distance. Some opticians will calculate this for you if you tell them how far away your monitor sits. A rough middle ground is subtracting 2 to 3 mm from your distance PD rather than the full 4 to 5 mm used for reading distance.
- Progressive lenses: Use your distance PD. The lab grinds progressive lenses so that the distance zone at the top aligns with your distance PD, and the reading zone at the bottom is inset to match the expected convergence. Entering your near PD for a progressive order would shift the entire lens incorrectly.
- Bifocals: Same as progressives. Supply your distance PD and the lab handles the near-segment inset.
When you order from an online retailer, the form usually asks for a single PD value. If the glasses are for distance, enter the distance number. If they are dedicated readers, enter the near number. Some sites offer a field for “near PD” or let you specify monocular values for each eye, which gives the lab even more precision.
Monocular Versus Binocular PD
Most people think of PD as a single number, but a more useful measurement splits it into two: the distance from the bridge of your nose to the center of your right pupil, and the distance from the bridge to the center of your left pupil. These are your monocular PDs. Very few faces are perfectly symmetrical, so your right and left monocular PDs often differ by a millimeter or more. A binocular PD of 64 mm might actually break down as 31.5 mm on the right and 32.5 mm on the left.
For mild prescriptions, using a single binocular number is usually fine. For stronger prescriptions, astigmatism corrections, or progressive lenses, monocular values make a real difference because the optical center of each lens can be placed independently. If your online retailer gives you the option to enter monocular PD for each eye, use it. If it only accepts a binocular number, adding the two monocular values together gives you the binocular total.
How to Figure Out Your Near PD If You Only Have the Distance Number
Eye care professionals routinely measure both values, but prescriptions do not always include PD at all, and when they do, it is usually only the distance measurement. If you have your distance PD but need the near number, a common approximation is to subtract 3 mm. That shortcut works reasonably well for an average adult. The research confirms it is in the right ballpark: measured differences between distance and near PD tend to fall between about 3.9 and 5.2 mm across a range of face sizes, with larger faces showing a slightly bigger gap.1Dove Press / Taylor & Francis (Clin Ophthalmol.). Influence of age on measured anatomical and physiological interpupillary distance (far and near), and near heterophoria, in Arab males So subtracting 3 mm is a conservative estimate. Subtracting 4 mm brings you closer to the measured average for most adults, and subtracting 5 mm may be more accurate if your distance PD is on the wider end (above 68 mm or so).
This is a workaround, not a replacement for a proper measurement. If you have a strong reading prescription and plan to spend hours a day in those glasses, it is worth getting the near PD measured directly.
Measuring PD at Home
If your prescription did not come with a PD value, you have a few options. The most accessible is having someone else measure you with a millimeter ruler while you look at a distant target. For near PD, you would focus on something about 40 cm (roughly 16 inches) away while they line up the ruler. The method is simple but introduces human error, especially if the person measuring is not practiced at it.
Smartphone apps have become a popular alternative. A comparative study of several measurement techniques found that the method used genuinely affects the number you get. For distance PD, mobile app measurements came out slightly shorter than pupillometer or ruler measurements. For near PD, the ruler tended to give higher readings than the pupillometer or mobile app.4Dove Press. A Comparative Analysis of Interpupillary Distance Measurement Techniques Evaluation in Modern Times: From Rulers to Apps The differences were statistically significant, which means the tool you choose can shift your PD reading by a millimeter or more in either direction.
Among smartphone apps specifically, a study comparing three popular options found that some performed considerably better than others. The Warby Parker app obtained accurate measurements most frequently across several precision thresholds, while the PDCheck AR app had a higher average error.5Cureus. Comparing the Effectiveness of Smartphone Applications in the Measurement of Interpupillary Distance If you go the app route, taking several readings and averaging them helps reduce random error. And keep in mind that most apps measure distance PD by default, since they show you a target on the screen at arm’s length. For a near PD measurement, you would need an app that specifically offers that option or a helper with a ruler.
Why Ready-Made Reading Glasses Often Feel Wrong
The cheap reading glasses you find at a drugstore are ground with a single, fixed optical-center distance, typically around 62 to 64 mm. That may be fine for an average male face at distance, but it is almost certainly too wide for comfortable near vision, where your pupils converge closer together. Research measuring ready-made readers against actual near PDs found a consistent mismatch. The discrepancy was particularly large for women, whose near PDs averaged about 59 mm compared to the 64.5 mm optical-center spacing of the readers designed for them.3CrossRef (African Vision and Eye Health). Analysis of readymade readers and near–inter-pupillary distance for presbyopic patients in optometric practice in Cape Town, South Africa
A separate study in Iceland estimated that about 14% of adults could not find standard supermarket reading glasses with an appropriate PD, even when prescription was not a factor.6Wiley Online Library. Refraction and interpupillary distance measurements of adult Icelanders with special reference to presbyopes possible use of supermarket standard spectacles If you have a narrow face, a wide face, or a higher reading add power, those off-the-rack readers are more likely to cause subtle discomfort that you might attribute to the “wrong prescription” when the real problem is the wrong PD.
For low-power readers (+1.00 to +1.50), the prismatic effect of a PD mismatch is usually small enough that most people tolerate it without symptoms. Once you get into +2.00 and above, the unwanted prism scales up and the odds of eyestrain go up with it. This is the range where custom reading glasses with your actual near PD start to pay off.
Does Prescription Strength Change How Much PD Matters?
It does, substantially. The prismatic effect from a misaligned optical center is proportional to the lens power. A 3 mm PD error in a -1.00 lens produces a very small prismatic displacement. The same 3 mm error in a -6.00 lens produces roughly six times as much prism. Research examining the relationship between decentration and prismatic effect using Prentice’s rule found that for spherical lenses, the errors involved are clinically insignificant at lower powers but become meaningful as lens strength increases.7PubMed Central. Spherical lens decentration errors by Prentice’s rule
This means if you have a mild prescription and you are choosing between distance and near PD for a pair of general-use glasses, the stakes are relatively low. But if your prescription is moderate to strong, getting the right PD for the intended use distance becomes one of the most important details of the order. People ordering high-power reading glasses online without a near PD measurement are the most likely group to end up with glasses that technically have the right prescription but still feel off.
PD Changes Over Your Lifetime
Your PD is not a static number from birth. It grows rapidly in childhood, particularly during the first five years, then continues to increase more slowly through adolescence before stabilizing in early adulthood.8Europe PMC. Growth Pattern and Use of Inter-pupillary Distance in the Detection of Ocular Hypertelorism and Hypotelorism in Indian Down Syndrome Children. Research tracking IPD changes over time has confirmed that the growth rate differs between boys and girls, with boys tending toward larger adult values, and that the biggest jump happens before age five with a much more gradual increase afterward.9PubMed Central. Changes in the interpupillary distance with age and its effect on the near fusion free position
For adults, the practical implication is that a PD measured in your twenties is probably still accurate in your forties. For children, however, a PD measurement can become outdated within a year or two, especially for young kids whose faces are growing quickly. If your child’s glasses were last fitted two years ago, it is worth having the PD re-measured rather than assuming the old number still applies.
There is also a subtle age-related factor for older adults. As the lens inside the eye stiffens with age (the reason you eventually need reading glasses), the effort of convergence can change slightly. The mechanical act of turning the eyes inward to read does not change your PD per se, but the near PD that matters for comfortable reading can shift by a millimeter or so as your convergence habits adjust over the years. This is another reason that fresh measurements matter more than old ones, especially when ordering near-vision corrections.
What to Do If an Online Retailer Only Asks for One PD
Many online glasses retailers have a single PD field. This leaves you to decide which number to enter. The answer depends on the lens type you are ordering. For distance single-vision, enter your distance PD. For progressive or bifocal lenses, enter your distance PD. For reading-only lenses, enter your near PD if you have it, or your distance PD minus 3 to 4 mm as an approximation.
Some retailers have caught up and now ask whether the glasses are for distance, intermediate, or near use, then apply the appropriate adjustment on their end. If the site you are using gives you that option, select the correct usage category and enter your distance PD. The lab will handle the inset. If the site does not ask about usage and just has a single PD box, you are responsible for entering the right value yourself.
A small but important detail: if you enter monocular PD values (like 31/33), make sure the retailer’s form actually accepts two separate numbers. Some forms add them together automatically and treat it as a binocular PD, which is correct. Others may misinterpret the input. When in doubt, add the two numbers yourself and enter the binocular total in a single field, keeping in mind that this sacrifices the precision of asymmetric placement.
When an Approximate PD Is Good Enough
Perfectionism about PD is not always warranted. For a pair of -1.50 distance glasses you wear to watch TV, being off by 2 mm produces a prismatic effect so small that you would never notice it. For non-prescription blue-light-blocking glasses or low-power sunglasses, PD barely matters because there is no meaningful lens power to create a prism. The people who benefit most from precise PD measurement are those with prescriptions above about ±3.00, anyone ordering progressive lenses where the corridor needs to be aligned carefully, and anyone ordering dedicated reading glasses at +2.00 or higher.
If you fall outside those categories, a standard measurement taken with a ruler or app is more than adequate. The anxiety around getting PD “exactly right” is understandable given how much money glasses cost, but for routine low-to-moderate prescriptions, a millimeter of error in either direction is well within the range your eyes can accommodate without complaint.