What Is Pupillary Distance (PD) and Why Does It Matter?

Pupillary distance, usually shortened to PD, is the distance between the centres of your two pupils, measured in millimetres. It tells the lens maker exactly where to place the optical centre of each lens so you look through the right spot. Get it right and your glasses feel natural; get it wrong and they can feel tiring or slightly "off".

What pupillary distance actually measures

Every lens has an optical centre. This is the point where light passes straight through without being bent sideways. When you look straight ahead, your pupil should line up with that point. Your PD is the measurement that makes this happen.

PD is not usually written as part of the main numbers on a prescription. Some optometrists include it, and some do not. If you want to understand the rest of your script, our guide on how to read your glasses prescription walks through SPH, CYL, AXIS and ADD step by step.

PD varies a lot from person to person. Adults often sit somewhere in the 50s to 70s, while children usually have smaller measurements that change as they grow. There is no "right" number. What matters is that the number is yours.

Single PD, dual PD and near PD

You may see PD written in a few different ways. They all describe the same idea, just in more or less detail.

Why dual PD is often more accurate

Very few faces are perfectly symmetrical. One eye may sit a millimetre or two further from the nose than the other. A dual PD captures that difference, so each lens is centred for its own eye. This becomes more important as prescriptions get stronger, and it is usually preferred for progressive lenses, where the reading and distance zones need to sit in the right place.

Near PD for reading

When you focus on something close, your eyes converge, meaning they turn slightly inwards. Your pupils move closer together, so a near PD is a little smaller than your distance PD. Single vision reading glasses are often made using the near PD.

Why an accurate PD matters

If the optical centres of your lenses are in the wrong place, you end up looking through the edge of the optics. This creates a small, unwanted effect called prism, which shifts the image slightly. Your eyes work harder to compensate.

A small error may not be noticed with a weak prescription. With a stronger prescription, the same error has more effect. People with an incorrect PD sometimes describe:

• Eyes that feel tired or strained after wearing the glasses for a while
• Headaches, especially later in the day
• A "pulling" or swimmy feeling when you first put the glasses on
• Difficulty settling into new lenses, particularly progressives

These symptoms can have many other causes, including an out-of-date prescription or general digital eye strain. If discomfort continues, see an optometrist or doctor rather than assuming it is just the PD.

How PD is measured

There are a few ways to measure PD. Some are more reliable than others.

1. Pupillometer. A handheld instrument used by optometrists and opticians. You look into it, and it measures each eye separately. This is quick and gives a dual PD.
2. Ruler measurement. A trained person holds a millimetre ruler across your face while you look at a fixed point. Done carefully, this works well.
3. Marking on the frame. For progressives, the optician may mark your pupil position directly on the demo lenses of the frame you have chosen. This also records fitting height.
4. At-home methods. Online apps and mirror-and-ruler tricks exist, but they can be off by a few millimetres. That may be fine for a mild single vision prescription, but it is not ideal for stronger or multifocal lenses.

Our honest recommendation is to have your PD measured by a professional. If your optometrist did not include it on your prescription, you can ask them for it, or ask us when you order your glasses.

PD, fitting height and frame choice

PD works together with the frame you choose. A frame that is too wide or too narrow for your PD can push the lens centres a long way from the middle of each lens. That can make lenses thicker on one side and affect how they look.

• Check the frame size numbers. Our guide to reading glasses size numbers explains lens width and bridge width.
• Stronger prescriptions often look neater in frames that suit your PD. See lens materials and thickness for why.
• For progressives and bifocals, fitting height (how high your pupil sits in the frame) matters as much as PD.

The way a frame sits on your nose also changes where your pupils land. If frames slip, the measurement is no longer quite right, which is one reason a good fit matters so much.

For more general information about eyes and vision, the American Academy of Ophthalmology has plain-language resources.

Need your PD measured or new lenses fitted? Visit our Panmure store, or bring your current frame in for a prescription update. In a hurry? Ask about same-day glasses, subject to prescription and stock.