What Is Pupillary Distance?

Medically Reviewed by Dany Paul Baby, MD on June 26, 2022
5 min read

Pupillary distance helps center your eyeglasses prescription. Your eyeglasses provider normally measures it for you, but you won’t find it on your prescription. If you order glasses online, you’ll need to get this measurement yourself. 

Pupillary distance, also called interpupillary distance or PD, is a measurement of the distance between your pupils. Pupils are the dark centers of your eyes that widen to let in light or narrow to focus on a close object. They help you see in the dark or when it’s too bright. Pupillary distance is one of the many measurements you need when you get glasses.  

There are two types of pupillary distance, and they can change depending on where you’re looking.

Far pupillary distance. This is the measurement from the center of one pupil to the center of the other when you’re looking off into the distance. Your provider will normally check this PD unless you’re buying computer or reading glasses.

Near pupillary distance. The near pupillary distance measures the distance from the center of your pupils when you focus your eyes on something close by, like when you’re reading. It’s usually about 3 to 4 millimeters shorter than your far PD. 

There are also two types of measurements. 

Single pupillary distance. Also called binocular pupillary distance, this measurement is the total distance from the center of one pupil to the center of the other. It’s called single because it’s only one number or binocular because it includes both eyes. It’s the most common method of measurement.

Dual pupillary distance. Also called monocular pupillary distance, this is the distance from the center of one pupil to the bridge of your nose. The dual PD has two numbers, one each for the left and right eye. It’s common to have one side that’s slightly different from the other. 

Pupillary distance is mostly used for sizing your glasses. Your eye doctor might use PD to check parts of your vision or eye function, but it’s not usually important for managing your vision treatment. That’s why it’s not on your prescription even though you need it when you’re ordering glasses.

Fitting glasses. For the best viewing experience, you want the center of your lens to sit right in front of your pupil. The center has the clearest part of your prescription, so lining it up with your pupil helps you see well and lowers eye strain. The measurement will change, though, depending on the type of glasses you buy.

An optician might use a handheld tool called a pupillometer or a digital PD meter. They hold this up to your face and adjust it to get the right distance. They might also use a special ruler to measure, or they might make marks on a pair of glasses and then measure the distance between them. 

Testing stereo acuity. Your eye doctor will use PD to understand your stereo acuity, which is your depth perception. Both eyes see images and objects individually, but your brain puts those left and right visions together to form a three-dimensional image. The pupillary distance helps your doctor understand how much your field of vision differs between your left and right eyes.

Testing near-point convergence. Near-point convergence occurs when your eyes focus together on a nearby object. Your doctor might test this by bringing a pencil or other object toward your nose and watching how well your eyes come together to focus. 

Your pupillary distance gets shorter as your eyes converge, or come together, so your doctor might check to see that your PD is changing. 

The pupillary distance is especially important for fitting reading glasses. 

The average pupillary distance for adults is 63 millimeters, though most adults range between 50 and 75 millimeters. Children usually have an average PD of at least 40 millimeters.

It’s important to measure your own distance and not to rely on averages, though. Everyone has a different pupillary distance, and it varies based on age, sex, and ethnicity. It also changes a lot with age, up to age 30. The most change happens from birth to age 19 as a child grows, though, so it’s important to remeasure when you’re getting a child new glasses.

If you’re ordering glasses online, you’ll need to measure your pupillary distance. Some companies have tools that can help you figure this process out, but it’s simple enough to do at home.

To measure your PD by yourself:

  1. Stand 8 inches away from the mirror.
  2. Hold a ruler up to your eyebrow. Make sure you are measuring in millimeters.
  3. Close your right eye and place the 0-millimeter mark at the center of your left pupil. 
  4. Open your right eye and look straight ahead. Then close your left eye. 
  5. Find the line that meets the center of your left pupil. This is your pupillary distance.

If you have trouble closing one of your eyes or you find it hard to measure by yourself, ask a friend or family member for help. To measure with a friend:

  1. Stand in front of your friend, about 8 inches away. Look straight ahead.
  2. Have them place the millimeter ruler on the bridge of your nose.
  3. Cover your right eye with your hand. Your friend will line up the 0 millimeters with the center of your left pupil. 
  4. Uncover your eye, look straight ahead, and then cover your left eye. 
  5. Your friend will line up the ruler with the center of your right eye. This is your pupillary distance.

Repeat the process 3 times and calculate the average of these measurements. This will help you get a more accurate distance. 

Pupillary distance makes your glasses prescription comfortable and clear. If you have the wrong measurement, it might cause blurriness, headaches, or eye strain. 

Mistakes can happen whether you measure it manually or your optician does it with a PD meter. Usually, that’s because the meter was too low on your bridge or you weren’t looking straight ahead. Still, it’s best to have a professional measure your PD, especially if you wear special lenses that need exact measurements, like progressive lenses

If you’re ordering children’s glasses, you struggle to get the measurement, or you’re nervous the number isn’t right, you can ask an eyeglasses provider or eye doctor’s office to measure it for you. Some might do it for free, while others might charge you a fee.