Learn · Your prescription, number by number

What is PD — pupillary distance?

PD is the distance between the centres of your pupils, in millimetres. It is not a measure of your eyesight at all — it is a fitting measurement, and it is the one most often missing from the paper you are handed.

What it is for

Every lens has an optical centre — the point where it bends light least and most accurately. That centre needs to sit directly in front of your pupil. If it is off by a couple of millimetres, the lens introduces a small unintended prism effect, which at low powers you may never notice and at high powers can cause eye strain, headaches or a subtle pulling sensation.

Typical values

Adults — roughly 54 to 74 mm, most commonly around 62 to 64 mm.
Children — smaller, and it grows with the face, so a child's PD needs re-measuring as they grow.
Near PD — about 3 mm less than the distance PD, because the eyes converge for close work. Reading glasses use the near value.

Binocular and monocular PD

A single number (say 63) is the binocular PD — pupil to pupil. Two numbers (31.5 / 32) are monocular — each pupil's distance from the centre of the nose. Faces are rarely symmetrical, so monocular measurements produce a better-centred lens, and they are what progressive lenses generally require.

Why it is often not on your prescription. In many countries the PD is treated as part of the dispensing measurement rather than the clinical prescription, so it stays with the optician who is making the glasses. You are usually entitled to ask for it — and it is worth keeping, because without it an online order becomes guesswork.

Measuring it yourself

A ruler against a mirror gets you within a millimetre or two, which is adequate for a low-power single-vision lens and not adequate for progressives or a strong prescription. If you are ordering anything complex, use a measurement taken by an optician.

It barely changes — so record it once

Unlike the rest of your prescription, an adult PD is essentially fixed. That makes it the ideal thing to store permanently rather than re-establish every time you buy glasses, and the reason a record that keeps it alongside the prescription saves a phone call later.

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