Getting your pupillary distance (PD) right is crucial for comfortable glasses. If you only have one measurement for both eyes, learning how to measure monocular PD yourself is a handy skill. This number tells the lens maker exactly where to place the optical center for each eye individually, which can be especially important for strong prescriptions or progressive lenses. While an optician’s measurement is always best, you can get a surprisingly accurate result at home with a few simple tools.
How To Measure Monocular PD
Monocular PD is the distance from the center of your nose bridge to the center of each pupil. It’s measured in millimeters. You’ll end up with two numbers—one for your right eye (RPD) and one for your left eye (LPD). These numbers might be equal, but often they are slightly different. Having both ensures each lens is perfectly aligned with your eye’s line of sight.
Why Monocular PD Matters More Than Binocular PD
Binocular PD is one number for both eyes together. It’s common, but it assumes your face is perfectly symmetrical. Monocular PD accounts for any asymmetry. Using monocular PD can lead to:
- Sharper vision, especially at the edges of your lenses.
- Reduced eye strain and headaches.
- Better performance in progressive or bifocal lenses, making it easier to find the right zone for reading or distance.
- A more natural and comfortable viewing experience overall.
What You’ll Need to Get Started
Gathering the right items before you begin will make the process smoother. You won’t need anything special.
- A millimeter ruler (one with clear, small markings is ideal).
- A well-lit mirror.
- A friend or family member to help (optional, but recommended for best accuracy).
- A pen and paper to record your numbers.
Method 1: The Self-Measurement Technique (Using a Mirror)
This is the most common DIY method. Follow these steps carefully for a reliable result.
- Stand about 8 inches from a mirror in good light. Hold the ruler flat against your brow, just above your eyes.
- Close your left eye. Align the ruler’s zero (0 mm) mark directly with the center of your right pupil. Keep your head and the ruler very steady.
- Look straight ahead. Without moving the ruler, open your left eye and close your right eye. The number on the ruler that aligns with the center of your left pupil is your binocular PD.
- Now, for monocular PD: Start over. Close your left eye again, with the zero at your right pupil’s center. Look straight at it in the mirror. Note the number directly above the center of your nose bridge (the indent between your eyes). This is your right monocular PD (RPD).
- Next, close your right eye. Move the ruler so the zero is now at the center of your left pupil. The number above the same center of your nose bridge is your left monocular PD (LPD). Write down both numbers seperately.
Method 2: The Friend-Assist Method (Most Accurate DIY)
Having a helper eliminates the need to move your focus and often gives a better result. Ask your friend to follow these steps.
- Sit or stand still, looking straight ahead at a distant object. Do not look at the ruler or your friend’s face.
- Your friend should hold the ruler gently against your brow. They must position themselves directly in front of you, so their perspective isn’t angled.
- They will close one of their own eyes to avoid parallax error. They then note the measurement from the center of your nose bridge to the center of your right pupil. That’s your RPD.
- Then, they measure from your nose bridge center to the center of your left pupil. That’s your LPD. They should repeat the process 2-3 times to ensure consistency.
Pro Tips for Avoiding Common Mistakes
Small errors can lead to an incorrect measurement. Keep these points in mind.
- Look at a distant target, not the ruler or the person measuring. Looking at something close will make your pupils converge and shrink the measurement.
- Keep the ruler level and straight. Any tilt can add extra millimeters.
- Measure multiple times. If you get 31/33, 31/32, and 30/33, your true monocular PD is likely around 31 for the right eye and 33 for the left.
- If your numbers seem off by more than 2mm from a previous measurement, take a break and try again later.
Understanding and Using Your Measurements
Once you have your numbers, you need to know what to do with them. A typical result might look like: RPD = 31mm, LPD = 33mm. Your total binocular PD would be 64mm. When ordering glasses online, look for a field that asks for “Monocular PD” or “Dual PD.” Enter each number in its corresponding box (Right and Left). If the form only asks for a single “PD,” add your two monocular numbers together and enter that sum.
When to Definitely See a Professional
Home methods are useful, but they aren’t perfect for everyone. You should get a professional measurement if:
- Your prescription is very strong (high myopia, hyperopia, or astigmatism).
- You are ordering progressive, bifocal, or computer glasses.
- You have a condition like strabismus (a turned eye).
- Your self-measurements are inconsistent or feel unreliable.
- It’s your first pair of glasses ever. An optician can double-check everything.
Most optometrists will provide your PD and monocular PDs upon request after an eye exam, sometimes for a small fee. It’s part of your prescription data.
FAQ About Measuring Pupillary Distance
What is the difference between monocular and binocular PD?
Monocular PD gives you two numbers: one for the distance from your nose to each individual pupil. Binocular PD is a single number measuring the distance between the centers of both pupils. Monocular is more precise.
Can I use a PD app on my phone?
Some smartphone apps use the front-facing camera to estimate your PD. They can be convienient, but their accuracy varies widely based on camera quality, lighting, and how you hold the phone. They are best used as a rough guide, not for definitive measurements for a new prescription.
How accurate do my at-home measurements need to be?
A 1-2 millimeter margin of error is usually acceptable for simple, single-vision lenses with a low prescription. For anything stronger or more complex, like progressives, you need to be within 1mm. This is why professional help is recommended.
What if my two monocular PD numbers are not the same?
That’s completely normal and very common. Few people have perfectly symmetrical faces. This is exactly why monocular PD is useful—it captures that assymetry so each lens can be correctly centered.
Is it hard to measure PD for reading glasses?
The process is similar, but your gaze changes. When measuring for reading glasses, you and your helper should look at an object about 14-16 inches away (like a book). This accounts for how your eyes converge for close work. Some opticians call this a “Near PD.”
Will an old pair of glasses have my PD on them?
Usually not. The PD is not engraved on the frames. However, an optician can sometimes measure the optical centers on your old lenses if they were made correctly, giving them a good estimate of your PD. It’s not always perfect if the glasses have slipped or been adjusted.
Measuring your own monocular PD is a straightforward process that can save you time and ensure your online glasses order fits well. By using a simple ruler and following the steps closely, you can get a measurement you feel confident about. Remember, for complex prescriptions or if you have any doubt, visiting an optician is the safest choice to guarantee clear, comfortable vision. With your accurate monocular PD in hand, you’re ready to order lenses that are truly customized for each of your eyes.