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How to measure pupillary distance (or PD)?
Medically reviewed by Alastair Lockwood on 28 September 2021
This article will show you how to measure your pupillary distance. You will need your PD measurement if you are buying glasses online.
What is pupillary distance?
Pupillary Distance (PD) measures the horizontal distance between the centre of your left and right pupils (the black parts in the middle of the eyes). This measurement determines the point where you look through the lens of your glasses. Pupillary distance ensures optimal clarity of vision.
Knowing your pupillary distance is crucial when it comes to buying a new pair of glasses online. It is particularly essential if you require a strong/complex prescription or varifocals as it won't be an option to use an average measurement for these lenses.
What is PD measured in?
A millimetre ruler or a PD ruler measures pupillary distance. The measurement is in millimetres. It is most often recorded as one value (e.g. 66mm) but can also be written as 33/33 or 34/32 (if you are measuring dual PD) depending on whether you are symmetrical.
Single PD or dual PD
Single PD and dual PD are methods for measuring pupillary distance.
Single PD, also referred to as binocular PD, is the measurement between the centre of one pupil to the other.
Dual PD, also known as monocular PD, measures the bridge of your nose to each eye. This method is the most accurate of the two.
How do I know my pupillary distance?
You can ask your eye care, professional or optometrist, for your PD measurement. While you will usually be given your pupillary distance during an eye examination, the law does not require them to give it to you.
Your dispensing optician should ensure that your glasses fit correctly and that the pupillary distance on your lenses is correct.
If you don't know your pupillary distance, you can check with your optician or measure it yourself at home with the help of a friend or the mirror.
How to measure your pupillary distance?
There are 3 ways to measure pupillary distance. You can measure your PD on your own with a using an old pair of glasses, mirror or with a friend.
You can use this millimetre ruler to measure your pupillary distance (don’t scale it up or down when you go to print as this is the exact size it needs to be.)
With A Mirror
You should follow the above steps and measure your pupillary distance a few times to ensure you get an accurate measurement.
A PD measurement between 58-68 mm is considered normal for men and women. The average PD for a man is 64mm, and for women, 62mm. For children, the average pupillary distance is between 43-58mm. If you’re unsure of your PD's select the average on our site, which has been set to 63mm
You can also measure your pupillary distance with help from a friend or using your eyeglasses.
1. Focus on an object in the distance with both eyes open.
2. Have your friend positioned just outside your peripheral vision at roughly the same height as you and get them to hold the ruler across your brow, resting on your forehead and the bridge of your nose.
3. Ask them to line up the ruler's 0 mm to your right pupil and measure across to your left pupil.
4. Make sure that they stay out of your line of sight. Keep looking straight ahead with your eyes as still as possible, and don’t look at your friend while they are measuring.
5. Do this with your friend a few times to make sure you get a consistent measurement.
How do you calculate near PD for reading glasses?
To calculate your near PD, subtract 3mm from your distance PD.
For example, if your measurement is 57mm, then your PD for reading glasses will be 54mm.
If you are calculating your near PD using your dual PD, you will only need to subtract 1.5mm from each eye's measurement.
What happens if PD is wrong on glasses?
If the pupillary distance is wrong on your glasses, this can result in eye strain, fatigue and blurry vision. A high prescription with the wrong PD can make these symptoms much worse. The wrong PD won't damage your eyesight; however, it will certainly be noticeable.
Note: A do-it-yourself PD calculated with a ruler at home will not be as accurate as the pupillary distance measurement you would get from your eye care professional. For the best glasses fit for your face and the distance between your eyes, schedule an eye exam.
Quick links:Glasses Frame Size Guide
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