How To Measure Your Pupillary Distance

You may have seen the term ‘PD’ used on your glasses prescription, with a number underneath. This stands for ‘pupillary distance’ and is all to do with getting the maximum power out of your glasses strength (so you can have the clearest vision). But how does it impact your clarity of vision, and what is it important? We’ll be going through all you need to know about pupillary distance and how understanding it can improve your vision.

What is Pupillary Distance?

This is the distance between the central points of your pupils, and is measured in millimetres. Your pupils are the round, black circles in the middle of your eyes that get bigger and smaller to let different amounts of light in. It’s these small dots that are considered the middle of your eyes.

They are surrounded by the iris, which is a ring of coloured tissue. It’s the iris that helps the pupil change size.

What is Pupillary Distance Used For?

A PD measurement helps accurately determine where on glasses lenses you’ll be primarily looking through. This is known as the optical centre. As we’ve mentioned, it’s the pupils that let light into your eyes, so having the optical centre lined up correctly on a pair of glasses is very important. So when PD is included on your prescription, your glasses will be made to line up the optical centre to the middle of your pupils.

PD, unlike the other parts of your prescription, is not an indication of your eye health. It’s a measurement taken to get the most out of your prescription, and so is not something to be concerned about.

Why is Pupillary Distance Important?

PD is important to get the most out of your glasses and your prescription. Aligning the optical centre accurately to your pupils is a key component in how glasses work. PD is used to make sure you clearly see things both up close and far away, depending on your prescription. If it’s not correct, your glasses may not improve your vision as much as they should. >

Without accurate PD, you may also be at risk of developing distorted and blurry vision, headaches, and eye strain. The likelihood of this is much higher if you have a stronger prescription. Your glasses may also feel uncomfortable to wear, and might not have the best fit. Pretty much the opposite of what glasses are intended to help with!

It should be noted that reading glasses will use PD a little differently. This is down to where individuals typically wear reading glasses, compared to everyday glasses. People who use reading glasses can wear them further down their nose, which means that the optical centre of the lenses may be different.

Average Pupillary Distance

The average pupillary distance for adults is between 54mm and 74mm. Broken down by gender, the average PD for women is 53mm to 65 mm, and 55mm to 70mm for men. PD is much smaller for children, where it is between 43mm and 58mm.

How to Measure Your Pupillary Distance

It is relatively straightforward to measure your PD at home, but you’ll want to have patience with it - it’s important for the measurements you take to be accurate.

To measure your pupillary distance, you’ll need a mirror, a ruler with millimetre measurements, and a steady hand.

Step 1- Stand around 30 cm away from the front of a mirror, ideally in a space that is bright. This will make it easier to see your measurements.

Step 2 - Take out your ruler, and hold it up horizontally by your eyebrows. Make sure you can see the measurement markings clearly.

Step 3 - Looking into the mirror, close your right eye and line up the 0mm mark on the ruler with the centre of your left pupil.

Step 4 - Still looking straight ahead into the mirror, close your left eye and open your right.

Step 5 - Whatever mm sits in the middle of your right eye’s pupil is your pupillary distance.

Step 6 - Don't forget to write it down!

Step 7 - We recommend repeating this process a few times, just to ensure that your measurements are accurate.

Step 8 - If you need a single PD, simply half the number you have measured.

How to measure somebody else's pupillary distance

Like measuring your own, you’ll need a ruler to measure someone else's PD. Follow the steps below, taking your time to measure them as precisely as you can.

Step 1 - Ask the person whose PD you are measuring to focus on something straight ahead in the distance, with both eyes open.

Step 2 - Using the ruler, line up the 0mm mark in the middle of one of their pupils.

Step 3 - You'll want to ensure that you don’t disrupt their sight line, as this can throw off the measurements. Try to measure at an angle, so you’re not directly standing in front of them.

Step 4 - Keeping the ruler straight, measure to the mm mark in the centre of their other pupil.

Step 5 - Repeat this process a few times to get the most accurate measurement, and write it down.

Step 6 - For a single PD measurement, just half the number you have recorded.

Got questions about measuring your pupillary distance, or want to know more about it? Contact us with any PD queries you have, and we’ll be happy to help.