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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 that 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:
- 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.
The researchers also state that this measurement can increase until a person is in their mid-20s. After this age, PD tends to remain constant.
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 pupil. 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 number 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.
There are two methods that people can use to measure their PD.
Method 1: Using a ruler
To use this method, people should stand about 8 inches away from a mirror. They should hold a ruler against their eyebrows and close their right eye to align the ruler’s 0 mm with the left pupil’s center.
While looking straight ahead, they can open their right eye, close the left one, and check the value on the ruler above the right pupil’s center. The resulting measurement is their 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 that is about 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 will need to measure from one pupil to the bridge of the nose, note the measurement, and then do the same for the other eye.
This process is not guaranteed to produce accurate results, so it is advisable to take several single and dual PD measurements and then find the averages.
Method 2: Using glasses
People who already wear eyeglasses can find their PD by putting their glasses on and asking another person to stand in front of them and mark two dots on the glasses right above the left and right pupils. Then, they can measure the distance between the two dots.
Once a person knows their single PD, they can calculate their near PD by removing 3 mm.
For example, a person 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 has not tested these products. All information is research-based.
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 here.
- 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 here.
- 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.
Pupillary distance 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.