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

Pupillary distance, usually shortened to PD, is one of the least understood numbers on a prescription — most people have never heard of it despite it being just as important to how well glasses actually work as the prescription strength itself.

What PD actually measures

PD is the distance in millimetres between the centres of your two pupils. It’s used to determine exactly where the optical centre of each lens needs to sit, so that when you look straight ahead, you’re looking directly through the part of the lens ground specifically for your prescription. It’s a simple measurement, but a precise one, and different for essentially everyone.

Why it matters as much as the prescription number

A correct prescription paired with an incorrect PD can still produce a lens that doesn’t work properly, because the correction is there but not positioned where your eyes actually look through it. This mismatch is a common, under-recognised cause of headaches, eye strain, and a general sense that new glasses “aren’t quite right” despite an accurate prescription — the numbers on paper are correct, but the lenses weren’t built around where your eyes actually sit.

Why it’s easy to get wrong without proper measurement

PD typically ranges from about 54 to 74 millimetres in adults and is measured slightly differently for distance vision versus close-up work, which is part of why some online glasses sellers ask customers to self-measure with a ruler and a mirror — a method that’s genuinely difficult to do accurately. A proper in-person measurement, usually taken with a specialised ruler or digital device during a fitting, removes that guesswork entirely, which is exactly why it’s worth having it measured properly rather than estimating it yourself.