Single PD vs Dual PD: Understanding Your Measurement Options

Single PD vs Dual PD

Last updated: January 2025 | Reading time: 7 minutes

When you look up PD measurement information, you'll encounter two formats: a single number (like 63mm) or two numbers (like 31/32mm). Both are valid ways to express pupillary distance, but they contain different amounts of information.

Understanding the difference helps you know which format you need and how to convert between them when necessary.

Definitions

Single PD (Binocular PD): The total distance from the center of one pupil to the center of the other pupil. One measurement, one number. Example: 63mm.

Dual PD (Monocular PD): Each eye's distance from the center of the nose bridge, measured separately. Two measurements, two numbers. Example: 31/32mm (right eye/left eye).

The Relationship Between Them

In theory, dual PD should equal single PD when you add the two numbers together:

If your dual PD is 31/32:

Right eye to nose bridge: 31mm
Left eye to nose bridge: 32mm
Total (single) PD: 31 + 32 = 63mm

In practice, this works out approximately, though minor measurement variations might make the math slightly off.

Which Contains More Information?

Dual PD contains strictly more information than single PD.

From dual PD, you can derive single PD (just add the numbers). From single PD alone, you cannot know the dual PD breakdown without making assumptions.

Example:

Knowing single PD = 64mm tells you the total distance.

But what's the left/right breakdown?

Could be 32/32 (perfectly symmetric)
Could be 31/33 (right eye closer to nose)
Could be 33/31 (left eye closer to nose)
Could be 30/34 (significant asymmetry)

Without measuring each eye separately, you can't know which scenario applies.

The Symmetry Assumption

When someone provides only single PD, optical labs typically assume symmetry—they divide by two and treat each eye equally.

64mm single PD becomes 32/32 monocular PD by assumption.

For people with symmetric faces, this works fine. For people with asymmetric faces, it introduces positioning error.

Research shows: About 85-90% of people have some degree of facial asymmetry affecting eye position. The average asymmetry is approximately 1mm difference between sides—small but measurable.

When Single PD Is Sufficient

For many situations, single (binocular) PD works adequately:

Low-to-Moderate Prescriptions

Weak prescriptions (under ±3.00) are less sensitive to positioning errors. The small prism induced by assuming symmetry when slight asymmetry exists rarely causes noticeable symptoms.

Simple Single Vision Lenses

Basic single vision lenses without specialized designs are more forgiving than progressive lenses or other complex formats.

Budget or Backup Glasses

For inexpensive glasses, casual wear, or backup pairs, the potential minor optimization from dual PD may not justify the extra effort.

When Monocular Isn't Available

If you can only obtain single PD (your eye doctor provided it, your measurement method only captures total distance), using that is better than guessing or using population averages.

When Dual PD Matters

Some situations strongly favor dual (monocular) PD:

Progressive Lenses

Progressive lens corridors are narrow and precisely positioned. Each eye needs its optical path correctly aligned. The stakes of assuming wrong symmetry are higher.

Most optical labs request monocular PD for progressives. If you provide only single PD, they'll assume symmetry—which may or may not be correct for your face.

High Prescriptions (±4.00 and above)

Stronger lenses create more prism per millimeter of decentration. The same 1mm asymmetry that's negligible at -2.00 becomes significant at -6.00.

Known Facial Asymmetry

If you're aware that your face is asymmetric—perhaps from photos, or because you've noticed one eye sits higher or further from center than the other—monocular PD ensures the asymmetry is properly accommodated rather than averaged away.

History of Glasses Problems

If you've had unexplained discomfort with previous glasses despite correct prescriptions, monocular PD might resolve subtle positioning issues you didn't know existed.

Premium or Custom Lenses

Freeform (digitally surfaced) lenses are individually calculated for your parameters. Providing monocular PD gives the algorithm more accurate data to work with.

How to Get Dual PD

Professional Measurement

Ask specifically for monocular PD. Digital pupillometers automatically measure each eye—the practitioner just needs to record both numbers rather than summing them.

If you're told "your PD is 63," ask: "What are the monocular values?" They likely have them.

Self-Measurement

Measuring monocular PD yourself requires identifying your nose bridge center and measuring from there to each pupil separately.

The nose bridge center is typically defined as the midpoint between the inner corners of your eyes (inner canthi). In a photo, you can mark this point and measure to each pupil.

Online Tools

Quality PD measurement apps and websites often provide monocular PD automatically. They detect both pupils and the nose bridge, calculating left and right distances separately.

Converting Between Formats

Single → Dual (Estimation)

If you only have single PD and need dual, dividing by two gives the symmetric approximation:

Single PD 64mm → Dual PD ~32/32mm (estimated)

This is an estimate only. Your actual dual PD might be 31/33 or 30/34 or anything else that sums to 64.

Dual → Single (Calculation)

If you have dual PD and need single, simply add:

Dual PD 31.5/32.5mm → Single PD 64mm (exact)

This conversion is mathematically exact—no estimation required.

Format When Ordering Glasses

Different retailers have different form formats:

Sites Asking for Single PD Only

Enter your single PD. If you only have dual, add the numbers first.

For most single vision lenses, this works adequately.

Sites Asking for Dual PD Only

Enter your dual PD. If you only have single, you can estimate by dividing by two—but understand this assumes symmetry that may not exist.

Consider getting proper monocular measurement if you're ordering progressives or have strong prescription.

Sites Offering Either Format

Choose based on what you have. Dual PD is preferable if available—it gives the lab more information without any downside.

Sites That Don't Clarify

Contact customer service to ask which format they expect. Entering 31/32 when they expect 64 (or vice versa) creates obvious problems.

Notation Variations

Dual PD can be written several ways:

31/32 — Slash between numbers
31.5 | 32.5 — Pipe or vertical bar
OD 31.5 / OS 32.0 — With eye labels (OD = right, OS = left)
R: 31.5 L: 32.0 — Alternate eye labels
32.0/31.5 — Some systems list left eye first (OS/OD order)

When providing dual PD, confirm which eye is which to avoid confusion. Right eye (OD) first is the more common convention, but verify with the specific retailer.

Practical Recommendations

Get monocular PD if you can. It contains more information and can always be converted to single PD if needed. The reverse isn't true.

Use monocular PD for:

Progressive lenses
Prescriptions over ±4.00
Premium lens designs

Single PD is acceptable for:

Basic single vision lenses
Moderate-or-weaker prescriptions
When monocular isn't available

When converting single → dual by dividing: Remember you're estimating. If the glasses don't feel right, asymmetry might be why.

Get both single and dual PD in one measurement. Our free online tool automatically calculates both formats—use whichever you need.