What Is Pupillary Distance?

What Is Pupillary Distance?

Pupillary distance (PD) measures the space between pupils in millimeters.

It’s used when fitting prescription eyewear. An eye doctor or optician takes your PD when getting new prescription eyewear. If you’re ordering a new pair of glasses online, you may need to measure your pupillary distance yourself. Fortunately, there are several ways to measure pupillary distance from home.

Why Is Pupillary Distance Important? 

Pupillary distance is used to fit prescription eyeglasses. Aligning the optical center of the lenses of your glasses results in greater comfort and visual clarity.

Proper PD measurement provides your eyes the best opportunity to focus. An accurate measurement drastically enhances the overall experience of wearing corrective lenses. 

Your eye care professional will measure PD as part of a routine eye exam and when your eye doctor prescribes eyeglasses.

What Happens if Your PD Measurement Is Off?

The further you drift from the optical center of your lenses, the more light will hit the eye's retina and bend it in various ways. 

Mistakes in PD measurements can cause the following problems:

• Headaches
• Eye strain
• Blurred vision
• Distorted vision
• Discomfort while wearing glasses 

Accurate PD measurements are especially important if you’re wearing glasses with special lenses, such as progressives.

How to Measure Your Pupillary Distance at Home

Most people have their PD measured when visiting their eye doctor and getting fitted for a new pair of glasses. It’s also possible to measure PD at home. There are different ways to do this:

How to Measure Pupillary Distance With a Ruler and a Mirror

1. First, stand 8 inches from a mirror and hold a millimeter ruler against your brow bone with your eyes open
2. Close your right eye and align the ruler’s 0 millimeter line with the center of your left pupil
3. Looking straight ahead into the mirror, close your left eye and open your right eye
4. The mm line that lines up with the center of your right pupil is the PD measurement

If you find this difficult to do yourself, you can have a friend or family member help you. Simply have a friend hold the ruler to the bridge of your nose. Then follow steps 2 to 4, having them move the ruler.

How to Measure Pupillary Distance With a Marker, Ruler, and Mirror

There is another method for measuring PD if you already wear prescription eyeglasses. You’ll need a non-permanent, felt-tipped marker.

1. With your glasses on, stand about 12 inches from a mirror.
2. Close your right eye and put a small mark on the left lens of your glasses in front of your pupil
3. Holding your head still, close your left eye and open your right eye
4. Looking straight ahead, place a small mark on the right lens as you did the left, right in front of your pupil
5. Remove your glasses and use a millimeter ruler to measure the distance between the two dots to determine your PD

You can compare the measurement you take to what you find on your prescription or your medical history. 

Virtual PD Measurement

The easiest way to measure pupillary distance at home involves using a virtual tool or digital PD ruler. Various websites and mobile apps offer this service to make it easier for you to buy new glasses online. 

Warby Parker is one example of an online glasses retailer that lets you measure your PD for free using their website or mobile app.

What Is the Average Pupillary Distance?

The average pupillary distance for adults is 63 mm. Most adults have a pupillary distance of 50 to 75 mm.⁴ However, PD is unique, and falling outside this range wouldn’t be considered abnormal. 

Single vs. Dual Pupillary Distance: What’s the Difference? 

There are two types of PD measurements: single (binocular) and dual (monocular) pupillary distance. Here’s how they differ:

Single (Binocular) PD

Binocular pupillary distance is the standard measurement of the total distance between the centers of both pupils. It’s called single PD because it’s one number, and binocular PD because it includes both eyes. 

Dual (Monocular) PD

Dual PD measures the distance between the center of one pupil and the bridge of your nose. If you're getting progressive lenses, your eye doctor or optician may measure your dual PD.

It’s called monocular PD because each number is the measurement for one eye. It’s called dual PD because there are two numbers, one for the left eye and one for the right eye. It’s common for the numbers to be slightly different in monocular PD measurements.

Near vs. Far PD

Pupillary distance is sometimes categorized as far or near PD. Unless you’re getting reading glasses, your eye doctor will probably measure far PD.

• Far PD. This measures the distance between the center of your right pupil and left pupil while you’re looking far away.
• Near PD. This is the distance between the center of each pupil when you’re looking at something up close. It’s typically 3 to 4 mm shorter than far PD.

Summary

Pupillary distance (PD) is used to measure the distance between your pupils. It’s expressed in millimeters (mm) and sometimes written on your glasses prescription next to PD.

Your optometrist or optician will measure your PD when you’re being fitted for prescription glasses. You can also measure your PD at home when buying new glasses online.

There are two types of PD measurements. Single (binocular) PD is the distance between the centers of both pupils. Dual (monocular) PD measures from the center of one pupil to the bridge of the nose. Single PD is more common.