Pupillary Distance

Pupillary distance (PD) is the measurement of how much space there is between the pupils in your eyes. It’s less common, but some eye doctors call it interpupillary distance (ID).

The pupil is the black center of your eye. Your PD measures the distance between them in millimeters (mm). Eye doctors need to know your PD to make lenses for your eyeglasses. They’ll use it to make sure the prescription in each lens line up correctly with the center of your eyes.

Your eye doctor may include your PD on your eye prescription they print out for you or include in your electronic health record. You’ll need to know your current vision correction prescription and PD if you want to buy glasses online.

Types of pupillary distance

There are two types of pupillary distance measurements:

• Single PD (binocular): This is the distance from the center of one pupil to the other. It’s written as a single number. This is the most common version of PD.
• Dual PD (monocular): This is a two-number version of your PD. It measures the distance from the center of your pupil to the bridge of your nose. Eye doctors write it like a fraction, with the number for your right eye before the left. For example, your dual PD might be 31/32.

Average pupillary distance

Your pupillary distance changes as your eyes grow and change during childhood. But it usually stays the same once you’re an adult and are done developing. Normally, adults have a single PD of around 63 mm. But it can range from 50 mm to 70 mm. It’s very rare to have a PD outside that range.

How to measure pupillary distance

An eye care specialist will usually measure your PD for you during an eye exam or when you need new glasses. But it’s possible to do it yourself. Your eye care specialist will give you the most accurate reading. They have professional tools that let them read your PD quickly.

Measuring your PD at home

If you want to measure your own PD, you’ll need a ruler marked with millimeter (mm) increments and a mirror. An optometrist or an employee at a store that sells glasses may give you a special ruler designed for measuring your PD. There are even smartphone apps that can help you get this measurement.

The accuracy of at-home rulers and apps can vary. You should be OK to measure your own PD at home. But having it done by a professional is always going to be the most accurate reading. See an eye care specialist for a PD measurement if:

• You need special lenses in your glasses, like bifocals, trifocals or progressive lenses
• You need glasses for close-up reading, as well as distance vision (your PD can change a tiny amount for these different tasks, so it’s extra important to get an accurate measurement)
• You have any condition like strabismus that can affect your eye alignment

How to measure PD by yourself

Follow these steps to find your own pupillary distance measurement:

1. Stand in front of a mirror in a well-lit space. Your face should be about 18 inches (46 centimeters) away from the mirror.
2. Hold the ruler to your face with the millimeter markings facing the mirror so you can see them. Hold it just under your eyes, like it’s a wall or fence you’re peeking over.
3. Close your left eye.
4. Using your right eye alone, line up the 0 mark of the millimeter scale underneath your right eye. You want the ruler’s 0 mark to line up with the center of your right pupil. Imagine a vertical line separating the pupil into left and right halves and line the 0 mark up with it. Don’t move the ruler on your face after this step, or you’ll need to start over.
5. Open your left eye and close your right eye.
6. Using your left eye only, find the nearest millimeter mark that lines up with the center of your left pupil. Again, it should be the mark that’s closest to the imaginary line that divides your pupil in half. That mark is the measurement for your PD.

How to measure PD with help

It’s easier to measure your PD with help. If you have someone to help you, the process is similar to doing it alone, but with a few tweaks:

1. Sit with your face about 18 inches (46 centimeters) away from your helper.
2. Hold the ruler to your face so they can see the millimeter marks. The ruler should be just under your eyes. Again, picture peeking over a wall or fence.
3. Look at your helper’s left eye. Keep your head still to get the best measurement.
4. Have your helper line up the 0 mark with the outer edge of your right pupil. If the ruler moves after this point, you’ll need to start over.
5. Look directly at your helper’s right eye.
6. Tell your helper to find the nearest millimeter mark to the inner edge of your left pupil. That measurement is your PD.