Measuring your pupillary distance

Pupillary distance (PD) is the gap between the centres of your two pupils, in millimetres. Most adults land between 54 and 74, with about 63 being typical.

It matters because a lens has an optical centre, and that centre is supposed to sit directly in front of your pupil. Get it wrong by a couple of millimetres on a weak prescription and you will never notice. Get it wrong on a strong one and the lens introduces prism: eye strain, headaches, and a vague sense that the new glasses are somehow not right that is very hard to put your finger on.

It is also the single most common reason an online glasses order goes wrong, because it is the one number that is not on your prescription slip. Opticians measure it at dispensing, and in the UK they are not obliged to write it down for you.

Three ways to get it

1. Ask your optician

Free, accurate, and they will usually just tell you. If you have had glasses dispensed there they already have it on file. Worth asking at your next test even if you are not buying.

2. Use your camera

The measuring page on this site reads it directly from your webcam, along with your face width and face shape, and the whole thing takes about two seconds.

The trick is that a photograph normally has no scale: a face could be small and close or large and far away, and pixels cannot tell you which. So you need something of known size in the shot. Traditionally that is a credit card held against your forehead. We use something you are already wearing: the iris.

Horizontal visible iris diameter is one of the most stable measurements in human anatomy. It is close to 11.7mm in every adult, regardless of age, sex or ethnicity, with a standard deviation of about half a millimetre. Face-tracking models that detect the iris boundary therefore hand you a ruler in the middle of the picture for free, and every other distance can be scaled off it.

In practice that gets you within a millimetre or so, which is fine for sunglasses, readers and moderate prescriptions. For a strong prescription, check it against one of the other two methods.

3. A mirror and a ruler

The old way, and it still works:

1. Stand about 20cm from a mirror.
2. Hold a millimetre ruler flat against your brow.
3. Close your right eye. Line the ruler’s zero up with the centre of your left pupil.
4. Without moving the ruler, open your right eye and close your left. Read off the number sitting over the centre of your right pupil.
5. Do it three times and take the middle value.

Easier with someone else holding the ruler, and easier again with a printed PD card.

Single PD, dual PD, and near PD

Sometimes PD is quoted as two numbers (say 31.5 / 31), which is the distance from the centre of your nose to each pupil separately. Faces are not symmetrical, and for a strong prescription or a varifocal that asymmetry is worth capturing. Two numbers added together give the single PD.

You may also see a “near PD”, which is a few millimetres smaller, because your eyes converge when you look at something close. It is used for dedicated reading glasses. If an order form asks for a distance PD and you have a near one, they are not interchangeable.

A caution

Measuring your own PD is entirely reasonable for a spare pair, sunglasses or a mild prescription. For a first pair, a strong prescription, varifocals or anything for a child, have it measured properly. The cost of getting it wrong is a pair of glasses you cannot wear.