What is Pupillary Distance? How to Measure PD?
What is my PD?
(also called Pupillary Distance, Inter-Pupillary Distance or Pupil Distance)
PD is the distance between the centre of one pupil to the centre of the other pupil. Prescription glasses are made so that the distance between the optical centres of the glasses' lenses, is the same as your PD. PD varies from person to person but once you are an adult, your PD does not change. Therefore, as an adult, if you have had it measured, then it will still be the same, even if your spectacle prescription has changed. Once you know your PD, you do not need to measure it every time you buy prescription glasses.
PD is measured either during the eye test or when prescription glasses are ordered. Obviously we cannot measure your PD over the internet, so it is best to make sure your PD is included in your glasses prescription.
The Diagram below represents a PD of 62mm.
If your prescription does not contain your PD:
- Ask to have it measured at an optical shop – some may charge a fee.
- Contact an optical shop where you have previously purchased prescription glasses. Your PD has been measured if you have ever had prescription glasses made up.
- Your previous optometrist may have a record of your PD.
- Alternatively, try measuring your own Pupillary Distance as explained below. (Note: Measuring your own PD may not be as accurate as having a trained person do it. If your head or the ruler moves during the measurement then the reading will be inaccurate)
The higher the lens power in the glasses prescription, the more important it is to use an accurate PD. These procedures may not work for everyone. We recommend that you obtain a professional measurement of PD whenever possible.
Measuring your own PD (Pupillary Distance):
TIP: This should be done with glasses off, if the numbers on a ruler are difficult to see, then a magnifying mirror may help.
1. Whilst looking into a mirror hold a ruler against the bridge of the nose with one hand.
2. Close your left eye, and line the '0' up with the centre of the pupil of your right eye as shown in the diagram.
3. Without moving your head or the ruler open the left eye and close the right eye. Read the number that lines up with the centre of the pupil of the left eye. This number represents your Distance PD in millimetres. You have just used this technique to measure your distance PD.
4. Repeat this whole process at least 3 times to try and get a consistent measurement in millimetres. Make sure your head and the ruler do not move after lining up the zero on the ruler until you take the reading. This procedure may be difficult if one eye has very poor vision compared to the other eye.
When you upload your prescription, enter the number you have just measured – this is your ‘distance PD.’ You then need to enter your ‘near PD’ which is calculated as 3mm less than the distance PD.
e.g. Distance PD = 60
Near PD= 60 – 3 = 57
Sometimes the centre of the pupil is difficult to see. Instead of lining up the ruler with the centre of the pupil, it is more precise to use the edge of the pupil as shown at the bottom of the page; (be careful to use the inside edge of one pupil and the outside edge of the other pupil.)
Getting a Friend to Measure Your PD:
This technique uses a second person to measure your Pupillary Distance (PD).
1. Both of you should be sitting down approximately 45cm apart. The person having their PD measured keeps both eyes open. When your friend is taking the measurement, they must keep one eye closed.
2. The ruler is held against the forehead as shown above. The person having the PD measured looks into the open eye of the person taking the measurement.
The “0” is lined up with the centre of one pupil. Record the number lining up with the centre of the other pupil in millimetres. This is the measurement for the “near PD”. Neither person should move their head during this procedure. This procedure will not work if the person having their PD measured has a turned eye. You have just used this technique to measure your near PD.
When you enter your PD details on the Stingy Specs glasses prescription form, enter the number you have just measured into the “near PD” field. Enter your “distance PD” which is calculated by adding 3mm to the “near PD.” (This rule is accurate for most people).
Eg Near PD= 57
Distance PD= 57 + 3= 60
Instead of using the centre of the pupils, you can use the edge of the pupils as shown below. (be careful to use inside edge of one pupil and outside edge of the other pupil.)
Please note that when you measure your own PD by using these techniques you may not get the same result as an experienced professional. We recommend that you use the measurements taken by an optometrist or an optical dispenser whenever possible. This page is provided for information purposes only. We do not take responsibility for the accuracy of measurements taken by people referring to this information. Please note that professional optical dispensers and optometrists may use different techniques to measure your PD.