Single or Dual PD—Which One Do You Need?
When you buy new eyeglasses, one of the most important measures taken is the distance between your pupils, also known as Pupillary Distance or PD. Just like shoes need the right size for comfort, eyeglasses must have the right PD measurement if they are to work properly. There are two ways to measure PD: take one total measurement across both eyes, known as Dual PD, or measure each eye separately, known as Single PD.
Pupillary Distance (PD): Single vs. Dual Measurements
Your pupillary distance (PD) is the space between your pupils, measured in millimeters. PD measurements come in two main forms: Single PD (also called Monocular PD) and Dual PD (also called Binocular PD). Both serve the same purpose but work differently for various eyewear needs.
Single/Monocular PD
Single PD measures the distance from the bridge of your nose to the center of each pupil separately. For example, your right eye might measure 31mm and your left eye 32mm, written as "31/32." This detailed measurement is particularly valuable for:
● High-power prescriptions
● People with asymmetrical facial features
● Complex vision corrections
Dual/Binocular PD
Dual PD is one measurement spanning the distance between both pupils. It's typically expressed as a single number, like "63mm." This simpler measurement works well for:
● Basic single-vision lenses
● Standard prescriptions
● Symmetrical face shapes
● Ready-made reading glasses
Key Differences at a Glance:
Feature | Single PD | Dual PD |
Format | Two numbers (e.g., 31/32) | One number (e.g., 63) |
Accuracy Level | More precise | Standard precision |
Best Used For | High prescriptions, progressives | Basic prescriptions |
Measurement Method | Individual eye measurements | Total distance measurement |
Cost to Measure | Usually higher | Generally lower |
Common Range | 29-35mm per eye | 58-70mm total |
How to Read PD on Your Prescription
Common PD Notation Formats
● Single Number Format: When you are reviewing your prescription, you may find a single number recorded in a format such as "PD 63" or simply "63." This reflects your Dual PD and is the total millimeter distance between your pupils. Doctors sometimes record this as "Distance PD: 63" or "B.PD 63," where "B.PD" is an abbreviation for Binocular PD. All these reflect the same meaning—complete distance across both pupils.
● Split Number Format: A split format is two numbers with a slash between them, like "31/32." The first number is your left eye measurement, and the second shows your right eye measurement. This may be written in more formal notation by doctors or other medical professionals, like "OD 31 / OS 32," where OD is your right eye and OS refers to your left eye. Both are saying the same thing—your measurements for each eye.
Where to Find PD on Your Prescription
PD measurements can appear in various places on your prescription form. Most commonly, you'll find it at the top or bottom of the prescription. Some doctors place it in a dedicated "Additional Information" section, while others include it in a specific box labeled "PD" or "Pupillary Distance." The location varies depending on the prescription format your doctor uses.
What If PD Is Not on My Prescription?
Many prescriptions do not come with the PD measurement, and that is okay. This is normal, as eye doctors often do not include PD because optical shops have to take their own measurements for accuracy in many cases. Some doctors keep PD measurements in their office records rather than on the prescription itself. If you need your PD measurement and it's not on your prescription, you can call your eye doctor's office or simply walk into an optical shop for a measurement. While online tools exist for self-measurement, they're generally less reliable than professional measurements.
Quick Reference Chart
Notation Type | Example | Meaning |
Single Number | "PD 63" | Total distance between pupils |
Split Numbers | "31/32" | Individual eye measurements |
OD/OS Format | "OD 31 OS 32" | Right eye/Left eye measurements |
With Distance Note | "Dist. PD 63" | Distance PD measurement |
All PD measurements use millimeters as the standard unit, with most adults falling between 58-70mm for total PD measurement.
How to Choose the Right PD for Your Glasses
Your ideal type of PD measurement depends on several key factors. Stronger prescriptions, especially over +/-4.00 diopters, may want more precise measurements for the best possible vision correction. The type of lenses also makes a difference: specialized lenses require more detailed measurements than basic lenses. Your facial symmetry plays a part, too, as some people are just naturally asymmetrical in their pupil positioning and therefore require individual eye measurements.
When Single PD Works Best
Complex vision needs require single PD measurements. If your prescription is stronger, such as +/-4.00 or higher, it's recommended that you make the measurements for each eye in order to position your lenses perfectly. Single PD measurement is a perfect application of Progressive Lenses because these progressive lenses have a multiple-point focus that aligns exactly with the eyes. These can benefit better Single PD measurements because of asymmetrical features in people—for example, if one eye is higher than the other.
When Dual PD Is Sufficient
With simple correction needs, Dual PD serves the purpose just fine. For single vision, basic prescriptions less than +/- 4.00 usually would not require the extra precision needed in measuring both individual eyes.
Professional Recommendations
Prescription Type | Recommended PD Type | Reason |
Strong (>+/-4.00) | Single PD | Higher precision needed |
Progressive/Multifocal | Single PD | Multiple focal points require exact alignment |
Basic Single Vision (<+/-4.00) | Dual PD | Standard precision sufficient |
Dual PD | Simple lens design | |
Asymmetrical Features | Single PD | Accounts for facial differences |
Most eye care professionals suggest getting your PD measured in person rather than relying on self-measurement, especially for complex prescriptions or specialized lenses.
How Experts Measure Your PD
What Tools Do Experts Use?
If you go to an eye care professional, they will measure your PD with highly technical machinery. The main gadget is something called a pupillometer—think of it as special binoculars that give super-precise readings of the distance between your pupils. They also check such measurements with simple but quite efficient tools: a PD ruler—a so-called PD stick—and digital modern devices with cameras able to make a shot of just exact measures of where the pupils of the eye are situated.
How Does the Measurement Work?
Your optician will first measure your "distance PD" by asking you to look at something far away—this help your eyes settle into their natural position. If you need reading glasses or progressive lenses, they'll also measure your "near PD" by having you focus on something close up. They will keep your head straight and steady throughout the process to get an accurate measurement; this usually takes only a few minutes in total.
Get the Most Accurate PD Reading Every Time
What to Watch | Why It Matters | How to Get It Right |
Head Position | Even slight tilts can throw off your measurement by 1-2mm | Keep your chin level and look straight ahead, just like taking a passport photo |
Room Lighting | Bright or dim light makes your pupils change size, affecting accuracy | Make sure the room has normal, steady lighting—not too bright or dark |
Where You Look | Your eye position shifts when looking at different spots | Focus on one specific point at eye level about 20 feet away (or as directed) |
Staying Still | Any head movement can lead to wrong measurements | Use the headrest provided and try to stay as still as possible for a few seconds |
Professional measurements typically achieve accuracy within 0.5mm, which is crucial for optimal vision correction. To ensure this precision, opticians often take multiple measurements and average the results.
Pro Tip: Think of it like taking a clear photo—the steadier you are, the better the results!
Quality Checks
Before finalizing measurements, professionals perform several verification steps:
● Compare measurements with standard ranges
● Take readings from different angles
● Cross-reference with previous measurements if available
● Verify measurements match facial symmetry
Get Your Perfect PD Measurement Today
Whether you need a Single PD for complex prescriptions or a Dual PD for basic lenses, it makes all the difference in how well eye glasses will work for you. As online measurements may seem so convenient, nothing replaces the accuracy of a professional measurement at your local eye care center. Set off right into perfect sight: schedule that professional PD through your eye-care provider at times when you get new glasses or an updated prescription for proper glasses. Your eyes are worthy of only the best, and proper PD creates the foundation for comfortable and truly effective eyeglasses.
