Single PD vs Dual PD (Monocular PD)

When you order glasses you may be asked for your pupillary distance — and sometimes for it in two different formats. The confusion between single PD vs dual PD trips up a lot of people, but the idea is simple once you see how the two relate. This guide breaks down what each number means and when you need it.

What single (binocular) PD measures

Single PD, also called binocular PD, is the total distance in millimetres between the centres of your two pupils. It is one number — for example 63 mm — and it describes your whole face in a single figure. This is the value most people picture when they think of pupillary distance, and it is what many online retailers ask for by default. If you are new to the topic, our overview of what pupillary distance is covers the basics.

What dual PD (monocular PD) measures

Dual PD — usually called monocular PD — splits that single number into two. Instead of measuring pupil to pupil, it measures the distance from the centre of your nose (the bridge) out to each pupil separately, one value for the right eye and one for the left. A typical result looks like 31.0 / 32.0, meaning the right pupil sits 31.0 mm from the nose bridge and the left sits 32.0 mm from it.

Why dual PD exists

Human faces are rarely perfectly symmetrical. The bridge of your nose is not always exactly halfway between your eyes, so each pupil can sit a slightly different distance from the centre. A single PD averages this out, but a monocular PD captures it precisely. That precision matters for centring lenses correctly — especially with progressive (varifocal) lenses and high-power prescriptions, where even a small offset moves the optical centre away from where your eye actually looks, causing eye strain or blur.

How dual and single PD relate

The two formats are not separate measurements — the dual values simply add up to the single one. In the example above, 31.0 + 32.0 = 63.0 mm, which is your single PD. So if you have an accurate monocular PD, you already have your binocular PD; you just sum the two halves.

How to read PD notation

When you see a notation like 31.5/32, the first number is almost always the right eye and the second is the left, following the optical convention of writing the right side first. Single PD appears as one figure, such as 63.5, while monocular PD appears as a pair separated by a slash. Values are given in millimetres and often to the nearest half millimetre.

When you need one number versus two

For everyday single-vision glasses, a single PD is usually enough. Ask for or provide your dual PD when you are buying progressive (varifocal) lenses, lenses with a strong prescription, or frames with a large or unusual shape, because precise per-eye centring makes a noticeable difference to comfort. Some retailers accept either format and convert as needed, while others specifically request both — when in doubt, give the two monocular values, since they contain the most information.

How this tool handles it

You do not have to choose. This PD tool reports both your total (single) PD and your left and right monocular PD automatically, so you can copy whichever format your retailer asks for. To learn the technique first, see our guide to how to measure pupillary distance.

Single PD vs Dual PD (Monocular PD), Explained