Every lens has an optical centre, and it needs to sit in front of the centre of your pupil. Pupillary distance — the gap between your pupils in millimetres — is what tells the lab where to put it. Get it wrong and lenses made to a perfectly correct prescription can still feel like hard work: eye strain, a slight pull, a headache by mid-afternoon.
It is also the number most likely to be missing from the sheet your optician handed you, because PD is a measurement for making glasses rather than a description of your eyes. Here is how to get it.
The most reliable route: ask
Your optician measured it when they made your last pair, and in most places is obliged to give you the prescription they wrote. Ask for the PD with it. This takes one phone call and beats every method below, because it was taken with a pupillometer rather than a bathroom mirror.
Failing that, read it off your last pair's paperwork or the case card some opticians include. If you find two numbers, see the dual PD section below — they are not interchangeable with one.
Measuring it yourself, with a friend
- Sit down. Have the other person stand about an arm's length away, at your eye level — looking down at you shifts the reading.
- Look straight past them at something far away, and keep looking at it. Focusing on their face pulls your eyes inward and shortens the measurement.
- They hold a millimetre ruler against your brow and line the zero up with the centre of your right pupil.
- Without moving the ruler, they read the millimetre mark at the centre of your left pupil.
- Repeat three times. If the three readings differ by more than a millimetre, do it again — one of them is a wobble, not a measurement.
Why the far-away thing matters
Your eyes converge for near work: the closer you look, the smaller the gap between your pupils becomes. A distance PD and a near PD genuinely differ, usually by around 3mm. Measuring while you stare at the ruler gives you a near PD and then uses it for distance lenses, which is precisely the error this whole exercise exists to avoid.
Measuring it alone, in a mirror
Stand about 20cm from a mirror, hold the ruler flat against your brow, close your left eye and line the zero up with the centre of your right pupil. Without moving your head or the ruler, close your right eye, open the left, and read off the centre of the left pupil. Do it three times.
It is the least accurate method here — you are working at arm's length from a reflection, so every small head tilt shows up. Treat it as a sanity check on a number you were given, not as the number you order with, especially above about −4.00 where a couple of millimetres start to matter.
Single PD and dual PD
| Written as | Means | Typical adult range |
|---|---|---|
| One number, e.g. 63 | The full distance between both pupils | 54 to 74 |
| Two numbers, e.g. 31.5 / 31 | Each pupil's distance from the centre of the nose bridge | 27 to 37 each |
Faces are not perfectly symmetrical, and a dual PD records that. If you have one, use it — adding the halves together and ordering a single PD throws away the asymmetry it was measured to capture. If you only have a single PD, that is normal and fine.
When to stop measuring and ask a professional
- A strong prescription. The stronger the lens, the more a misplaced optical centre costs you.
- Progressives. They also need a fitting height, which cannot be measured without the chosen frame on your face.
- Prism in your prescription. The alignment is the whole point of the lens; do not estimate its reference point.
- Children. Small faces, small margins, and a child cannot tell you the lenses feel subtly wrong.
Once you have it, the rest of the fit is frame geometry — the numbers printed inside your glasses covers how to read that off a pair you already own. Then browse eyeglasses, or send us the prescription and we will tell you if anything looks off before you order.
Written by
Eyevora Editorial
Product & care desk at Eyevora. We stock the frames we write about, and we publish every measurement we hold. Spotted something wrong here? Tell us and we will correct it.
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