How to Measure Pupillary Distance (PD)?

If you wear glasses or are a new glasses wearer, you may have heard of the term ‘pupillary distance’ or PD but do you what it actually is? Let’s answer all your questions around PD and learn tried-and-tested ways to measure it comfortably at home.

What is pupillary distance and why is it required?

Pupillary distance is the measurement from the centre of one pupil to the centre of the other pupil It ensures the lenses of your glasses sit in the correct position in front of your eyes to allow optimum visual clarity. Accurate PD measurement makes wearing glasses throughout the day a comfortable experience, significantly minimising eye strain or headaches. That is why, it is important to know how to measure pupillary distance correctly before ordering glasses.

Here are a few key things to know about PD:

• It is measured in millimetres.
• It is usually written as a single number, such as 63 mm.
• The measurement helps align the optical centre of the lenses with your eyes.
• This ensures that light enters the eye and focusses on the retina, when you wear glasses.

Note: Most adults have PDs between the range of 54mm to 74mm.

Single PD vs dual PD

A single PD (also called binocular PD) measures the distance between both pupils, denoted in just one number. Usually used for standard prescriptions and single vision orders.

A dual PD (also known as monocular PD) splits the measurement into two numbers. One number measures the distance from the centre of the nose bridge to the centre of left pupil and the other measures the same distance to the centre of right pupil. Dual PD can generally be used for:

• Varifocal or bifocal lenses
• Higher prescriptions
• When you need extra accuracy

Why is measuring pupillary distance important?

Pupillary distance measurement is important because it affects how your lenses line up with your eyes.

How to measure PD at home?

You do not need professional equipment to measure pupillary distance. There are a few simple ways to measure your PD at home, such as:

1. Using a mirror and a ruler

This is one of the most common ways people measure PD at home. All you need to do is:

• Stand at an arm’s length away from a mirror to clearly see both eyes.
• Hold a ruler with millimetre markings straight across the bridge of your nose.
• Close one eye and line up the zero mark with the centre of the open pupil.
• Switch eyes and read the measurement at the centre of the other pupil.

This method works best if you keep the ruler level and repeat the measurement a few times.

2. Asking a friend to help

• Ask your friend to hold the ruler across the bridge of your nose.
• Look straight ahead at a point in the distance to keep your eyes steady.
• Measure from the centre of one pupil to the centre of the other.

This method often gives a more stable measurement, especially if you struggle to keep the ruler still.

3. Using an old pair of glasses

If you already wear glasses and have an old pair, it is much easier to calculate. Just:

Put the glasses on and look straight ahead into a mirror, keeping your head level.

Using a felt-tip pen or washable marker, place a small dot on one lens, marking your pupils.

Repeat the same on the other lens.

Take the glasses off and use a ruler to measure the distance between the two dots.

If you require reading glasses, you need a near PD, as your eyes move slightly closer together when focusing on nearby objects.

To calculate near PD:

• Subtract 3 mm from your distance PD.
• For example, If your distance PD is 57 mm, your near PD would be about 54 mm.
• If you are using a dual PD, subtract 1.5 mm from each eye’s measurement.

Pupillary distance measurement might seem like a small detail, but it has major impact on how your glasses feel.

Taking a few minutes to measure pupillary distance properly can help avoid discomfort later. If you are unsure about your measurement, professional PD measurement is always an option. When measured correctly, pupillary distance supports clear and comfortable vision.