How to Measure Your Pupillary Distance (PD)

Why is it important?

Your PD measurement is used in the alignment of your glasses to make sure your pupils are lined up with the centre of each lens. For optimal vision, it’s important that you are looking through the central part of the lens, so you need to make sure you measure pupillary distance accurately.

Single vs. dual PD

People sometimes talk about two different types of PD measurement: single and dual PD. Single PD is the distance between the centres of the two pupils. Dual PD is a measurement that consists of two numbers: the distance between each of the pupils independently and the bridge of the nose. The first number of the dual PD measurement is always the right eye and the second is the left eye.

How to measure your own PD in 5 simple steps

1. Stand facing a mirror, at a distance of about 20cm.
2. Lay the ruler flat horizontally across the mirror in front of you and close your right eye.
3. Line up the 0 mark of the ruler with the centre of your left pupil and keep your head still.
4. Without moving the ruler, close your left eye and open your right one.
5. Now read the millimetre measurement that lines up with the centre of your right pupil.

Measure from the sides of the irises

If you’re having trouble deciding where the centre of the pupil is, you can measure from the far-left edge of the iris (the coloured circle around the pupil) of one eye to the far-left edge of the iris of the other. This measurement works out the same as from the middle of one pupil to the other.

Don’t feel like you can get an accurate reading by measuring yourself? Here’s how a friend can measure your PD.

1. Your friend should sit at the same level as you and hold a millimetre ruler across the bridge of your nose (making sure the measurement markings on the ruler are on the side closest to your eyes).
2. Your friend should close their left eye and then use their right eye to line up the centre of your left pupil with the 0 mark on the ruler.
3. Without moving the ruler, they must then open their left eye and close their right eye.
4. Then your friend should read the measurement that lines up with the centre of your right pupil.

How do you calculate near PD for reading glasses?

If you are looking at an object close up—for example, when you’re reading—your pupils tend to move closer together. So when you are calculating the distance between your pupils for the purposes of reading glasses (a measurement known as near PD), you need to adjust the figures.

In order to arrive at your near PD measurement, you need to subtract 3mm from the regular PD measurement (known as a distance PD). To calculate a near dual PD, subtract 1.5mm from each of the two measurements.