Pupillary Distance

What is pupillary distance?

Pupillary distance (PD) is the measurement of the distance between the center of the pupils of your eyes. It’s a key measurement that eye care providers and professionals will use for making prescription eyeglasses. Experts sometimes call it interpupillary distance (ID).

Measuring your PD is possible on your own, but it’s much better to have someone help. You’ll need your PD measurement and current vision correction prescription if you want to buy eyeglasses online.

If you want to measure your PD, all you need is a ruler marked with millimeter (mm) increments and a mirror. Special rulers designed for PD measurement are available online from various retailers. There are even smartphone apps that can help you get this measurement.

But the accuracy may vary if you use a smartphone app or take the measurement on your own. Seeing a trained eye care professional is best if you need to be certain your PD measurement is accurate.

Measuring your own pupillary distance

To measure your PD on your own with a millimeter-increment ruler, do the following:

1. Stand in front of a mirror in a well-lit space. Your face should be about 18 inches or so away from the mirror.
2. Hold the ruler to your face, millimeter markings facing the mirror so you can see them. Hold the ruler just under your eyes, like it’s the edge of a wall or fence you’re peering over the top of.
3. Close your left eye.
4. Using your left 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). It’s important not to move the ruler on your face after this, 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 most closely lines up with an imaginary dividing line that separates your pupils into equal left and right halves. That mark is the measurement for your PD.

Measuring your pupillary distance 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 adjustments.

The measurer should do the following:

1. Sit with your face about 18 inches from the face of the person you’re measuring.
2. Hold the ruler to their face so the millimeter marks are visible. The ruler should be just under their eyes (like they’re looking at you over a wall or fence).
3. Have the person you’re measuring look specifically at your left eye. Try to keep your head still from this point on.
4. Line up the 0 mark with the outer edge of their right pupil (remember, their right is your left). Make sure to keep the ruler still after this or you’ll need to do it again.
5. Have the person you’re measuring look specifically at your right eye.
6. Find the nearest millimeter mark to the inner edge of their left pupil. The nearest millimeter mark is the measurement for their PD.

Why does pupillary distance matter?

The pupils are where light enters your eyes. After passing through the pupil, light travels through the lens, which bends light so it reaches your retinas. But if you have a vision condition, that bending may not work correctly, giving you blurry vision at certain distances. Examples of this include:

• Farsightedness (hyperopia).
• Nearsightedness (myopia).
• Presbyopia (age-related trouble with focusing on objects up close).
• Astigmatism.

There’s no one-size-fits-all approach when it comes to prescription eyeglasses. That’s because of the variables that can affect your prescription, especially the exact vision issue you have, the strength of your prescription and any degree of rotation in your vision (if you have astigmatism).

Taking the PD helps providers correctly align the light-bending properties of prescription lenses with the pupils of your eyes. That way, you get the maximum benefit from the lenses.

Nasopupillary distance

PD isn’t the only number that matters. There’s also the nasopupillary distance, which is the distance between one pupil and the center of the bridge of your nose. About 4 out of 5 people don’t have symmetry of their eyes and nose, meaning their nasopupillary distance isn’t the same for both eyes.

For people who don’t have eye-nose symmetry, the distance is usually small, less than 2 mm. But a little over 1 in 6 people have a difference of more than 2 mm. While that distance is small enough that most people don’t notice it (or might not even be aware of it), it can still affect how prescription eyeglasses (especially multifocal lenses) work.

What happens if my pupillary distance measurement isn’t accurate?

If you’re wearing glasses made using an incorrect PD measurement, you’ll probably notice symptoms related to the misalignment. Those include eyestrain, headaches and distortions in your vision. These symptoms aren’t dangerous, but they also aren’t pleasant. And the additional strain on your eyes can also cause your eyesight to worsen.

Does pupillary distance change?

Your pupillary distance changes as you grow during childhood, but once you become an adult, it usually doesn’t change.

But there are rare circumstances where your pupillary distance might change. An example of this would be an eye or face issue requiring surgery. Reconstructive surgery might slightly alter your pupillary distance.

What’s the average pupillary distance?

The average adult pupillary distance is about 63 mm. But the most likely range of adult pupillary distances can be as small as 50 mm and as large as 70 mm. A small proportion of adults has a pupillary distance outside that range. They can be as small as 45 mm or as large as 80 millimeters.

When should I not rely on my own pupillary distance measurement?

If you only use eyeglasses for one purpose, like reading or fine detail work, your pupillary distance should be consistent for the use of those glasses. But there are a couple of reasons why a precise measurement of your PD might be necessary, including:

• If you need multifocal lenses (progressive lenses, bifocals or trifocals). Accurate PD is critical to correctly making each segment of these lenses.
• If you need to wear glasses constantly for both up-close work and seeing at a distance. That’s because your pupillary distance gets slightly narrower (by about 3 mm or so) when looking at things up close.
• If there’s any amount of misalignment in your pupils (strabismus).

Why is my PD not on my prescription?

In the United States, there’s a rule that all providers who write an eyeglasses prescription must give you a legibly written or printed prescription at no extra charge. The U.S. Federal Trade Commission enforces this, and it’s informally known as the “Eyeglass Rule.”

The Eyeglass Rule doesn’t specifically include PD as part of the prescription. But some states have laws that require your prescription also include the PD. At least one state, Arizona, previously required it but later amended the law so it’s no longer required. As of 2018, the states with PD-specific laws include:

• Alaska.
• Kansas.
• Massachusetts.
• New Mexico.

You may want to check to see if your state has enacted laws about pupillary distance inclusion on your prescription.

If you’re not in one of those states, your provider isn’t required to give you the PD for free. They may give it to you if you ask, but some providers may charge an extra fee.

A note from Cleveland Clinic

The pupillary distance is a measurement of the distance between the two pupils of your eyes, and it might not be something you’ve given much thought to. But if you wear glasses, that measurement can make a big difference in the quality of your vision. You can measure your own PD, but it’s generally best to have a professional do it. A difference of just a millimeter or two can turn corrective eyewear into an unhelpful — or even disruptive — accessory.