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Pupillary distance is the space — measured in millimeters (mm) — from the center of one pupil to another. When you need glasses, an optometrist or an optician will measure this distance to make sure your eyeglasses have the best function and fit for you. If you are ordering eyeglasses online, you may need to measure your pupillary distance, or PD, yourself. Keep reading to learn how.

Simply stated, pupillary distance (PD) is the measurement of the distance between your two pupils, and it helps make sure you eyeglasses will fit and align correctly with the center of your eyes. This alignment and how the glasses are positioned on your face is important when comparing the function of glasses used for seeing things that are far away versus seeing things that are close to you.

Eyeglasses that are not fitted with the correct PD measurement can lead to concerns like:

  • eye strain
  • headaches
  • distorted vision
  • blurred vision
  • difficulty wearing your glasses

If you are ordering eyeglasses online, there are several ways to measure your PD from home.

With a ruler

If you have a ruler and a mirror or helper, you can measure your PD at home.

  1. Get a ruler. Some websites offer printable rulers, too.
  2. Stand in front of a mirror and hold the ruler horizontal. Keep it stable.
  3. Start with the ruler lined up to the center of one pupil and measure the distance to the next pupil.
  4. This number is your PD.

If there is someone who can help you, it might be easier to have someone else measure your PD for you. If you need multifocal lenses, you should also measure the individual distances from the center of each pupil to the bridge of your nose.

Take your measurement several times to make sure it’s accurate. Be sure to look straight ahead while taking the measurement.

Without a ruler

If you don’t have a ruler, the online supplier for your glasses may offer a printable option. You can also cut a piece of paper or cardboard and mark the center of your pupils, then measure that distance if you are having trouble using a ruler and mirror.

Other ways to get your PD

A 2014 study showed that some websites offer technology that will measure your PD for you. There are apps or programs to take a photo of your face and measure your PD that way.

You can also call your optometrist or eyeglasses provider and ask for a copy of the measurement.

There are a few specifics when it comes to how your pupillary distance is measured. Your gaze naturally shifts inward or outward depending on whether you are looking at things close up or far away. For this reason, Eye Influence says that your optometrist or an optician may measure two types of PD — far and near.

  • Far PD. This is used for all types of glasses, except reading glasses or glasses that are used for computer work only.
  • Near PD. This measurement measures your PD when your gaze shifts inward, like when you are looking at things up close. This is mostly used for things like reading glasses, and the measurement can change by 3 to 4 mm compared to your Far PD.

Eye Influence also says that your measurement may change based on what types of eyeglasses you need.

  • Binocular PD. For single vision glasses, you will have a single number that measures the distance from the center of one pupil to another.
  • Monocular PD (from pupil to bridge). For progressive, bifocal, or trifocal lenses, you will have two numbers. Each one measuring the distance from a pupil to the center of your nose. This is because each eye may have different needs. The total of these separate measurements should equal the single number measurement.

Your optometrist or optician will know that your pupillary distance needs to be measured differently. If you are ordering eyeglasses online, you should be asked for additional measurements based on your prescription.

While PD is an important measurement when it comes to the fit and function of your glasses, it’s not technically a part of your vision prescription or eye examination. Your eye examination will be performed by an optometrist in most cases, and your PD may be measured by an optician — the provider that creates the right eyeglasses to fit your prescription.

Measuring PD is especially important when you need multifocal lenses like bifocals. In this case, you may also need to measure vertical distances and more to find the right fit. In many cases, the measurements needed to ensure the right fit for your eyeglasses are set by the manufacturer of those lenses.

When you are ready for a new pair of glasses, you or the provider who is creating your eyeglasses, will take several measurements to make sure the fit and function are appropriate for your needs. Pupillary distance measurements check the distance between the center of each pupil, and it’s important to measure this distance correctly to prevent complications like headaches and blurred vision.

If you are ordering glasses online, you will need to take this measurement yourself or with the help of a friend. Be sure to take the measurement several times to make sure you have the most accurate result.