Monocular PD vs Binocular PD: Which Do You Need and Why It Matters

Monocular vs Binocular PD

Last updated: January 2025 | Reading time: 10 minutes

You've probably seen pupillary distance written two different ways: as a single number like "64mm" or as two numbers like "31.5/32.5." Both are valid PD measurements—but they're not interchangeable, and using the wrong one for your glasses can cause real problems.

The difference between monocular and binocular PD seems technical until you understand what it actually means for your vision. This guide breaks down when each measurement type matters and how to ensure you're using the right one.

The Basic Distinction

Binocular PD (also called single PD or total PD) is the total distance from the center of one pupil to the center of the other pupil. It's expressed as a single number: 64mm.

Monocular PD (also called dual PD or individual PD) measures each eye's distance from the center of your nose bridge. It's expressed as two numbers: the right eye (OD) measurement followed by the left eye (OS) measurement. Example: 31.5/32.5 or OD 31.5 / OS 32.5.

Simple enough. But here's what many people don't realize: these measurements reveal different information about your face, and that information becomes critical in certain situations.

The Asymmetry Problem

Human faces are rarely perfectly symmetrical. Look closely at almost anyone's face, and you'll notice slight differences between the left and right sides. The eyes are no exception.

Research consistently shows that approximately 85-90% of people have some degree of facial asymmetry affecting eye position. A 2006 study using 3D facial scanning found that the average asymmetry in pupil position was 1.1mm—meaning one eye sits noticeably further from the nose center than the other.

For some people, this asymmetry is minimal: maybe 0.5mm difference between sides. Their monocular PD might be 32/32.5, essentially symmetric.

For others, the asymmetry is significant: 2mm or more. Their monocular PD might be 30/33—a substantial difference that binocular PD alone can't capture.

Here's the key insight: if your monocular PD is 30/33, your binocular PD is 63mm. That single number tells the lab to position optical centers 63mm apart total, but it doesn't tell them where to position them relative to the frame center. They'll likely assume symmetry (31.5/31.5), which is wrong for your face.

The result: one lens is positioned 1.5mm too far from center, the other 1.5mm too close. Your eyes are looking through slightly off-center zones of both lenses.

When Binocular PD Is Sufficient

For many glasses, particularly basic single vision lenses with moderate prescriptions, binocular PD works fine. Here's when you can confidently use just the single number:

Low-to-Moderate Prescriptions

If your prescription falls between -3.00 and +3.00 diopters with minimal astigmatism, small positioning errors cause minimal prismatic effect. Your eyes can compensate without strain.

At -2.00 diopters, a 1.5mm decentration per eye creates only about 0.3 prism diopters—usually imperceptible.

Symmetrical or Near-Symmetrical Faces

If your monocular PD differs by less than 1mm between eyes (like 31/32), the error from assuming symmetry is small enough that most people won't notice.

Non-Critical Use Glasses

Backup glasses, reading glasses for occasional use, sunglasses—situations where you're not wearing the glasses all day or demanding peak optical performance.

When You Don't Know Your Monocular PD

If you can't obtain monocular measurements and need glasses quickly, binocular PD is better than nothing. The resulting glasses may not be perfect, but they'll generally be functional.

When Monocular PD Becomes Essential

Certain situations demand monocular PD for proper lens fabrication:

Progressive Lenses (Multifocals)

Progressive lenses have narrow corridors of clear vision for different distances. The fitting is precise: the distance zone must align with your straight-ahead gaze, the reading zone with your downward reading position.

If the optical centers are placed asymmetrically relative to your actual pupil positions, the corridors won't align with how your eyes naturally move. You'll struggle to find the clear zones, especially for intermediate (computer) distance.

Progressive lens adaptation complaints correlate strongly with fitting errors, including PD errors. One major study found that 60% of progressive lens "failures" could be attributed to fitting issues rather than intolerance to the lens design.

High Prescriptions (+/-4.00 and above)

As prescription strength increases, the consequences of decentration grow proportionally. At -6.00 diopters, that same 1.5mm decentration now creates 0.9 prism diopters—significant enough to cause headaches and eye strain for many wearers.

If your prescription is strong and your face is asymmetric, using binocular PD essentially guarantees suboptimal glasses.

Prism Prescriptions

If your prescription includes prism correction (for eye alignment issues), precise optical center positioning is critical. The prism is ground into the lens at specific locations; if those locations don't match your pupil positions, you get the wrong effective prism.

Occupational Lenses

Specialty lenses for specific work tasks—computer glasses with enhanced intermediate zones, pilot glasses with specialized layouts—require precise positioning to function as designed.

High-End Digitally Surfaced Lenses

Modern "free-form" or "digital" lenses are individually calculated for each wearer. The manufacturing process assumes precise positioning data. Using approximate PD wastes the potential of these premium lenses.

Sensitive Patients

Some people are simply more sensitive to optical imperfections. If you've had trouble adapting to glasses in the past, or if you notice subtle visual issues others might ignore, monocular PD can make a meaningful comfort difference even in situations where it's theoretically "optional."

How to Obtain Monocular PD

Professional Measurement

The most accurate monocular PD comes from professional measurement using a digital pupillometer. These devices independently locate each pupil and provide separate readings.

When getting measured professionally, specifically request monocular PD. Don't assume you'll receive it—many practices still default to binocular measurement only.

Self-Measurement

Measuring monocular PD yourself is trickier than measuring binocular PD because you need to establish your nose bridge center as a reference point.

The nose bridge center isn't arbitrary—it's conventionally defined as the point equidistant from the inner corners of your eyes (the inner canthi). In a photo or mirror, you can identify this point and measure from there to each pupil center.

Some online PD tools automatically calculate monocular PD using facial landmark detection. These tools identify your nose bridge and measure each eye independently.

Deriving from Binocular PD

If you only have binocular PD, you can estimate monocular PD by dividing by two—but this assumes symmetry. 64mm binocular becomes 32/32 monocular as an estimate.

This estimate is fine if your face is reasonably symmetric. If you know or suspect significant asymmetry, the estimate isn't reliable.

Reading Your Measurements

Monocular PD can be written several ways:

31.5/32.5 — Right eye first, then left
OD 31.5 / OS 32.5 — With eye designations
R: 31.5 L: 32.5 — Alternative notation

Some systems record left eye first (OS/OD order). Confirm which convention is being used to avoid confusion.

The two values should sum to approximately your binocular PD. If your monocular PD is 31.5/32.5, your binocular PD is 64mm.

What the Numbers Tell You

Beyond fitting glasses, your monocular PD reveals something about your facial structure:

Symmetric Monocular PD (within 0.5mm)

Your eye positions are essentially equal relative to the nose bridge. You have average facial symmetry in this dimension.

Moderate Asymmetry (0.5-1.5mm difference)

Slight but measurable asymmetry. This is normal and common. Monocular PD becomes more valuable for progressive lenses and strong prescriptions.

Significant Asymmetry (more than 2mm difference)

Noticeable asymmetry. For any prescription work, you'll benefit from using monocular PD. Binocular-only fitting may never feel quite right.

Note that asymmetric PD doesn't mean anything is wrong with you—it's simply a normal variation in human facial structure. But it does mean your glasses need to accommodate your specific geometry.

Practical Recommendations

Based on everything above, here's straightforward guidance:

Always Use Monocular PD For:

Progressive lenses
Prescriptions stronger than +/-4.00
Any lenses with prism
Premium free-form/digital lenses
If you have known facial asymmetry

Monocular PD Recommended For:

Prescriptions between +/-2.00 and +/-4.00
Computer glasses or occupational lenses
If you're sensitive to optical issues
If previous glasses haven't felt right despite correct prescription

Binocular PD Usually Sufficient For:

Prescriptions under +/-2.00
Basic single vision lenses
Reading glasses (without progressives)
Backup or occasional-use glasses
If your face is reasonably symmetric

If Uncertain:

Use monocular PD when available. It never hurts—the lab simply has more precise positioning data. The only reason to use binocular PD is if monocular isn't available.

The Cost Difference

Getting monocular PD professionally measured typically costs the same as binocular measurement—the pupillometer captures both automatically.

Using monocular PD when ordering glasses doesn't usually affect price either. The lab needs to position optical centers regardless; they just have more precise instructions.

The real cost is getting it wrong: glasses that don't work well, adaptation struggles, remaking lenses, or simply living with suboptimal vision because you didn't know better.

Bottom Line

Think of binocular PD as the average and monocular PD as the details. The average works in many situations, but when precision matters—and it matters more often than people realize—the details make the difference.

If you don't know your monocular PD, find out. It's a one-time measurement that remains valid for years, and having it available ensures you can always get glasses made to your exact specifications.

Your face is unique. Your glasses should accommodate that uniqueness, not assume you're "average."

Get both your binocular and monocular PD in one measurement with our free online tool. Takes 30 seconds, no signup needed.