Blog · 18 Aug 2026

How to measure your pupillary distance at home

PD is the distance between the centres of your pupils, in millimetres. Every online optician asks for it. The lenses are cut so their optical centres sit exactly in front of your pupils. If the number is wrong, you look through the wrong part of the lens.

An optician measures PD in seconds. Many charge for it, and many leave it off your prescription. You can measure it at home instead. Here are three ways, from most accurate to least.

First, know what a normal PD looks like

Use this table as a sanity check. If your measurement falls outside these ranges, measure again.

Two more terms you may meet. A single PD is one number, eye to eye. A dual PD is two numbers, one per eye, measured from the bridge of your nose. Dual PD matters when your face is asymmetric, which most faces are, a little.

Method 1: an iPhone depth scan (about ±1 mm)

Face ID iPhones carry a TrueDepth camera. It measures your face in three dimensions, in real millimetres. An app that reads this camera does not guess your scale from a photo. It measures it.

SpecFit does this free. You make one guided scan. The app draws your PD, face width, bridge width, and brow line on your own photo, so you can see what was measured and where. The result is accurate to about ±1 mm. It is an estimate, not a medical measurement, and it does not replace an optician's exam.

No Face ID? SpecFit can estimate your PD from one clear selfie instead. A photo estimate is less exact than a depth scan. The app labels it honestly as an estimate.

Method 2: a mirror and a ruler (about ±2-3 mm)

You need a mirror and a millimetre ruler.

1. Stand about 20 cm from the mirror.
2. Hold the ruler flat against your brow, numbers facing the mirror.
3. Close your right eye. Line up the ruler's zero with the centre of your left pupil.
4. Hold still. Open your right eye and close your left.
5. Read the millimetre mark at the centre of your right pupil. That number is your PD.

Repeat it three times and take the middle value. The hard part is holding the ruler still between steps. Expect an error of 2 to 3 mm.

Method 3: a friend and a ruler (about ±2 mm)

Sit down and look at a point at least 3 metres behind your friend. Your friend holds the ruler across your brow and reads pupil centre to pupil centre. Looking far away matters: it keeps your eyes from converging, which would shrink the number. Two careful passes usually agree within a millimetre or two.

What about credit-card apps?

Some apps ask you to hold a credit card against your forehead. The card's known width gives the photo a scale. It works, roughly. The card sits forward of your eyes, so perspective adds error, often 2 mm or more. A depth camera makes the trick unnecessary.

When accuracy matters

For a mild single-vision prescription, a millimetre or two of error is usually fine. Two cases deserve better. Strong prescriptions magnify centring errors into visible distortion and eye strain. Progressive lenses stack three zones into one lens, so the centres must sit exactly. For those, use the depth scan or ask your optician for the measured number.

SpecFit measures your PD free. One guided scan on a Face ID iPhone, accurate to about ±1 mm, drawn on your own photo. The same scan powers a photoreal try-on of any glasses you find.

Questions

Is my PD written on my prescription?

Sometimes. Many opticians leave it off. Look for a number between 54 and 74, often labelled PD. If it is missing, measure it yourself or ask for it. In many places you are entitled to it.

Does a 2 mm error matter?

For a mild prescription, usually not. For strong prescriptions and progressives, yes. See the section above.

What is the difference between near PD and distance PD?

Distance PD is measured while you look far away. Near PD is measured while you focus close. Near PD is about 3 mm smaller, because your eyes converge to read. Reading glasses use near PD. Everything else uses distance PD.

Can I reuse the PD from my old glasses?

Your PD does not change in adulthood. If an optician measured it for your current glasses and the glasses feel right, that number is still good.