Pupillary Distance (PD): What It Is and How to Measure It

Pupillary distance, usually shortened to PD, is one of the most important measurements in making a pair of glasses, yet it rarely gets much attention. It is simply the distance between the centres of your two pupils, measured in millimetres. That single number tells the lens lab where to position the optical centre of each lens so that you look through the part of the lens that gives the clearest, most comfortable vision.

If you buy glasses from an optician, the PD is measured for you as part of the fitting. If you order glasses online, however, you are often asked to provide it yourself. This guide explains what PD is, the difference between single and dual PD, how it is measured both professionally and at home, and why accuracy matters so much more for some lenses than others.

What PD is and why it matters

Every spectacle lens has an optical centre, the point where light passes straight through without being bent sideways. When you look through the optical centre, you get the intended correction with no unwanted side effects. When you look through a point away from the centre, the lens also acts as a weak prism, shifting the image slightly. This is known as induced prism.

A small amount of induced prism is tolerated easily, but larger amounts force the eyes to turn slightly inwards or outwards to keep the images aligned. Over hours, this can cause eye strain, headaches, a pulling sensation, difficulty reading or even double vision. The stronger your prescription, the more prism a given centring error creates. This relationship is described by Prentice's rule: induced prism (in prism dioptres) equals the decentration in centimetres multiplied by the lens power in dioptres.

This is why a PD that is a couple of millimetres out may not matter for a mild prescription but can be a real problem for a strong one. Our guide on how to read your glasses prescription explains the other numbers on your prescription, including prism that is prescribed on purpose.

Single versus dual PD

Single (binocular) PD

Single PD is one number, such as 63 mm, measured from the centre of one pupil to the centre of the other. It assumes that both eyes are the same distance from the centre of the nose. For many single-vision glasses with mild prescriptions this is perfectly adequate.

Dual (monocular) PD

Faces are rarely perfectly symmetrical. Dual PD measures the distance from the centre of the bridge of the nose to each pupil separately and is written as two numbers, for example 31.0/32.5, with the right eye listed first. The sum gives the single PD (63.5 in this example). Monocular PD allows each lens to be centred precisely on its own eye and is the standard for progressive lenses, bifocals and higher prescriptions.

Distance PD and near PD

When you look at something far away, your eyes point almost straight ahead and the distance between your pupils is at its maximum. When you read, both eyes turn inwards (converge) to look at the same point, so the pupils move closer together. Near PD is therefore smaller than distance PD, typically by about 3 to 4 mm at a normal reading distance.

Single-vision distance glasses use distance PD. Reading glasses made for one specific near distance use near PD. Computer glasses use a value in between. For progressives and bifocals, the lab uses the distance PD and builds in the inward shift of the reading zone automatically according to the lens design. If you are ordering reading glasses and only have a distance PD, tell the supplier; they can calculate the near value.

Typical values

Most adult PDs fall approximately between 54 and 74 mm, with many people somewhere in the low-to-mid sixties. Children have smaller PDs, which increase as they grow, often starting in the low to mid-forties in young children. Because children's PDs change, they should be remeasured each time new glasses are made. The value has no bearing on how good your eyesight is; it is purely an anatomical measurement.

How PD is measured

By an optician

Opticians use several methods. The simplest is a PD ruler: the optician sits at your eye level, closes one eye while you look at their open eye, and reads the measurement. More accurately, a pupillometer is a handheld device that you look into while it measures each monocular PD using corneal reflections. Many practices now use digital centration systems that photograph you wearing your chosen frame and calculate monocular PD, fitting heights and other parameters such as frame tilt and wrap.

Measuring PD at home

If you need your PD for an online order and cannot get it from your optician, you can measure it at home with reasonable accuracy for simple single-vision glasses.

1. Stand about 20 cm in front of a mirror in good light, or ask a friend to help, which is more accurate.
2. Hold a millimetre ruler flat against your brow, just above your eyes.
3. Close your right eye and line up the zero mark with the centre of your left pupil.
4. Without moving your head or the ruler, close your left eye and open your right eye. Read the mark at the centre of your right pupil.
5. Repeat three or four times and use the average.
6. If a friend measures, look at a point about 3 m away over their shoulder so you get a distance PD, and keep the ruler level.

Many online retailers also offer apps or webcam tools that estimate PD using a reference object such as a credit card. These can be helpful, but accuracy varies with lighting, camera angle and how you hold the reference card.

Why PD matters so much for progressive lenses

Progressive lenses have a narrow corridor in the middle of the lens where the power changes smoothly from distance to near, flanked by zones of unwanted blur and distortion. For the lens to work properly, this corridor must line up exactly with the path your eye travels when you look down to read. Even a small horizontal error means you look through the edge of the corridor rather than its centre, which narrows the clear field and makes you turn your head awkwardly to find the sweet spot.

For this reason, progressives require:

• Monocular PD rather than a single PD, because each eye's corridor must be centred individually.
• Fitting height (segment height), the vertical distance from the pupil centre to the bottom of the lens opening in your chosen frame. This determines where the distance zone ends and the corridor begins.
• Frame measurements and adjustment, since the PD is only meaningful once the frame sits correctly on your face.
• Sometimes additional parameters such as vertex distance, pantoscopic tilt and frame wrap for personalised designs.

Fitting height cannot be measured without the actual frame on your face, which is one reason many professionals recommend buying progressives in person. If you do order them online, choose a supplier that offers a professional fitting service or accepts measurements from a local optician. Our guide to lens types explains how progressives work, and the article on adapting to progressive lenses covers the first weeks of wear.

Signs your PD may be wrong

If your PD is incorrect, you may experience some of the following, particularly with a strong prescription or new progressives:

• Eye strain, aching or a pulling sensation, especially after reading.
• Headaches that improve when you take the glasses off.
• Blurred or doubled vision, or a feeling that images do not merge comfortably.
• With progressives, a very narrow reading area or the need to turn your head to one side to read.
• A sense that the floor is tilted or curved.

Not every complaint with new glasses is caused by PD. An adaptation period is normal, especially with a significant prescription change or a new lens type. However, if symptoms persist beyond about two weeks, return to the optician. They can check the lens centres against your measured PD and confirm whether the glasses were made correctly. Frame selection also matters: a frame that is far too wide or narrow for your PD may require lenses to be heavily decentred, which our frame guide explains.

PD and other eyewear

PD is also relevant for binoculars, microscopes and virtual reality headsets, which have adjustable interpupillary distance settings for comfort. Contact lenses do not need a PD, because they move with the eye and are always centred over the cornea.

When to see an eye care professional

Measuring your PD is a fitting task, not an eye examination. If you have ongoing headaches, double vision, eye strain that does not improve with correctly made glasses, or difficulty getting your eyes to work together, an optometrist or orthoptist should assess you for binocular vision problems, which can sometimes be helped with prism or supervised vision therapy. Our page on eye examinations explains what to expect.