Monocular PD vs Binocular PD: What's the Difference?
What is split PD and single PD, why your face isn't symmetrical, and which measurement you should submit for your glasses.
When filling out an online form to buy prescription lenses, you will usually be asked to enter either a single number (e.g., 63) or two separate numbers (e.g., 31.5 and 31.5). These represent your Binocular and Monocular Pupillary Distances.
Binocular PD is the total distance in millimeters from the center of one pupil to the center of the other.
- • Format example: 63 mm
- • Assumes both eyes are perfectly centered on the nose bridge.
- • Common for single-vision lenses and mild prescriptions.
Monocular PD measures the distance from the middle of your nose bridge to each individual pupil center.
- • Format example: 31 / 32 mm (written as Right/Left).
- • Accounts for natural facial asymmetries.
- • Essential for progressive lenses and strong scripts.
Understanding Facial Asymmetry
No human face is perfectly symmetrical. Your nose bridge may sit slightly to one side, or one eye may be positioned a fraction of a millimeter wider than the other.
If your total Binocular PD is 64 mm, your monocular split is rarely a perfect 32/32. It is highly common to have measurements like 31 mm (Right) and 33 mm (Left).
If you fit a single vision frame using a generic 32/32 split, your left eye will look slightly through the off-center part of the lens, introducing subtle distortions.
When Should You Use Monocular (Split) PD?
You should always try to use Monocular PD if:
- You are ordering Progressive Lenses:Progressive lenses contain distinct corridors for distance, intermediate, and reading vision. If the lens is off-center by even 0.5 mm, your focal corridors won't line up with your eyes.
- You have a High Prescription: Stronger powers (+/- 4.00 diopters or more) have greater refractive power. A misaligned center creates a prism effect, leading to eye strain and headaches.
- You are ordering Reading Glasses: When you read, your eyes naturally converge (rotate inward). Monocular reading PDs are typically 1.5 - 2 mm narrower per eye than distance PDs.
Calculate Both Instantly
Our free AI-based online sizer calculates both your Binocular PD and Monocular splits (Right/Left) using vector coordinates.