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The pupillary distance (PD) measurement is a requirement for people who want to purchase eyeglasses. Some eyewear manufacturers provide how-to videos to help customers calculate their PD without visiting an eye doctor’s clinic.

This article describes what PD is and explains the different measuring methods individuals can use.

PD refers to the distance in millimeters (mm) between the centers of the pupils in the eyes.

This measurement helps manufacturers prepare prescription glasses, as it ensures that the prescription aligns with the person’s eyes. PD indicates exactly where the individual will look through the lenses of their glasses.

There are two types of PD. Far PD refers to the distance between the pupils when a person is viewing a distant object. Eye doctors use this type of PD for distance vision glasses. Near PD is the distance between the pupils when an individual is looking at a near object, such as when reading.

It is important to measure PD correctly. The Canadian Association of Optometrists states that inaccuracies in PD measurements can lead to:

  • eyestrain
  • headaches
  • distorted vision
  • blurred vision

According to a 2016 study, PD can vary among individuals. The researchers note that various factors, including age, sex, and ethnicity, can influence the measurement.

For instance, research has shown that males typically have a higher PD than females, with various studies reporting an average PD of 60.14 to 69.97 mm for males and 57.33 to 66.68 mm for females.

The researchers also state that this measurement can increase until a person’s mid-20s. After this age, PD tends to remain constant.

The authors of a 2016 study concluded that age plays a significant part in PD. Older individuals generally have a smaller PD than younger persons or those with myopia.

There are no studies investigating the average PD for children. However, some companies that sell eyeglasses use a default measurement if a parent or caregiver does not know the child’s PD.

Most children’s PD is 43–58 mm, and eye care professionals can measure this distance during an eye exam.

There are two types of PD measurements: single PD and dual PD.

Single or binocular PD measurement refers to the distance between the center of one pupil and the center of the other. A single PD measurement provides one number, such as 60, 63, or 55 mm.

Dual or monocular PD refers to the distance between each pupil’s center and the bridge of the nose. Dual PD measurements include two numbers. For example, if the dual PD is 31/33, the first number would refer to the right eye, and the second would refer to the left eye.

The American Academy of Ophthalmology notes that some prescriptions may also include the abbreviation OD or OS. OD is short for oculus dexter, which refers to the right eye, and OS is short for oculus sinister, which refers to the left eye.

Learn how to read an eye prescription.

A person can measure their PD by using a ruler. Here is a step-by-step guide on doing this:

  1. Stand up straight, about 8 inches from a mirror.
  2. Hold a ruler against the eyebrows and close the right eye to align the ruler’s 0 mm marker with the left pupil’s center.
  3. While looking straight ahead, open the right eye, close the left one, and check the value on the ruler above the right pupil’s center.
  4. The resulting measurement is the single PD.

Individuals can also measure their single PD with the help of another person. The individual who requires a measurement can look at an object approximately 20 feet away from them. The person measuring can align the ruler’s 0 mm mark with the center of one pupil and then measure to the other pupil’s center.

To calculate a dual PD, people need to measure from one pupil to the bridge of the nose, note the measurement, and then do the same for the other eye.

People should take several single and dual PD measurements and calculate the average distance to ensure a more accurate result.

People who already wear eyeglasses can find their PD by following these steps:

  1. Put the glasses on.
  2. Ask another person to stand in front of them.
  3. Using a non-permanent marker pen, mark two dots on the glasses above the left and right pupils.
  4. Measure the distance between these two dots with a ruler.

This measurement is a person’s PD. People may wish to perform these steps several times to ensure a more accurate result.

Once a person knows their single PD, they can calculate their near PD by removing 3 mm.

For example, someone with a single PD of 60 mm would have a near PD of 57 mm.

Alternatively, a person can use their dual PD and subtract 1.5 mm from each eye’s measurement. For someone with a dual PD of 31/33 mm, the result would be 29.5/31.5 mm.

It is important to know what a person’s PD is to ensure that their glasses fit correctly. An optician can measure PD when fitting glasses. However, a person who buys glasses online may need to measure their PD at home.

Please note that the writer of this article has not tried these products. All information presented is purely research-based and correct at the time of publication.

Medical News Today follows a strict product selection and vetting process. Learn more here.

People can choose from a range of companies that sell glasses online. The following companies offer a service that allows a customer to see how frames look on them before they commit to a purchase. They also have a generous returns policy in case a person measures their PD incorrectly.

  • Warby Parker: This company allows people to try on up to five frames for free at home. Warby Parker also offers a 30-day refund or exchange policy and a 1-year no-scratch guarantee on their lenses. Learn more about Warby Parker.
  • Zenni Optical: This company has a virtual try-on service that allows people to get an idea of which frames would suit them. Zenni Optical also offers a 30-day returns policy for store credit. Learn about Zenni Optical.
  • EyeBuyDirect: This company allows people to upload photos and take a quiz to decide which frames may be most suitable for their face shape. EyeBuyDirect offers a 14-day fit and style guarantee with a one-time replacement or refund. Its glasses also come with a 365-day product guarantee that allows customers to claim a one-time replacement pair if the glasses are defective. Learn more about EyebuyDirect.

Here, we answer some frequently asked questions about measuring PD.

Is there an app to measure PD?

There are several apps available to measure PD, such as GlassesOn, available on the App Store and Google Play.

People can also ask an eye care professional to measure their PD during an eye exam.

Can you measure PD from old glasses?

Yes, a person can measure their PD using their old glasses. They will need another individual to help them by placing two dots on the glasses above the left and right pupils.

People can then measure the distance between these dots.

What happens if my PD measurement is off?

If a person does not have a correct PD measurement, it can affect the usefulness of glasses, especially progressive lenses.

Someone who is unsure of their PD measurement may wish to contact an eye care professional to accurately measure the distance between their pupils.

PD measures the space between a person’s pupils to ensure that their glasses and lenses fit correctly. Opticians will measure an individual’s PD when they are fitting glasses. Alternatively, a person can measure their PD at home.

People who purchase glasses online may wish to buy from companies that have a generous returns policy in case they mismeasure their PD, which would make their glasses less effective.