Your pupillary distance (PD) is the distance between the centres of your pupils, in millimetres. You need it to order glasses online, because it tells us where to place the optical centre of each lens. You can measure it at home in about five minutes with a millimetre ruler and a mirror, or with help from someone else.
If you want the background on why PD matters, read What PD Is and Why It Matters When Ordering Glasses Online. This guide is the practical how-to.
Check whether you already know your PD
In the UK, your PD is not part of your glasses prescription. The College of Optometrists describes it as a dispensing measurement, taken when glasses are fitted, not during the eye test. That’s why it is often missing from the prescription you’re given.
It’s still worth asking the practice where you had your eye test. If they measured it when fitting a previous pair of glasses, they may have it on file.
What you need to measure your PD
- A ruler with millimetre markings. A flat plastic ruler is easier to hold than a tape measure.
- A mirror, for the solo method.
- Good, even light, so you can see your pupils clearly.
- A pen and paper to note each reading.
Three ways to measure your PD at home
Choose the method that suits you. Measuring with a friend is usually the most accurate.
Method 1: Ruler and mirror (on your own)
- Stand about 20cm from a mirror, in good light.
- Hold the ruler flat against your brow, just above your eyes, with the millimetre edge running horizontally.
- Keep your head still and look straight ahead at your reflection.
- Close your right eye. Line up the zero mark with the centre of your left pupil.
- Keep the ruler still. Close your left eye and open your right.
- Read the measurement in line with the centre of your right pupil. That number is your PD.
- Repeat the whole process three times and use the average.
Because your eyes are focused on your reflection, a mirror measurement can come out slightly narrower than your true distance PD. Method 2 avoids this.
Method 2: With a friend (more accurate)
- Sit or stand facing your friend, with your eyes at the same height, about an arm’s length apart.
- Look past them at something at least 3 metres away, such as a far wall. Looking into the distance keeps your eyes in the position used for distance glasses.
- Your friend holds the ruler just below your eyes, level and still.
- They line up zero with the centre of your right pupil and read the mark at the centre of your left pupil.
- Repeat three times and average the results.
Tip: The centre of a pupil can be hard to judge. Your friend can measure from the outer edge of one pupil to the outer edge of the other instead. Because both pupils are the same size, the result is the same as centre to centre.
Method 3: Use our virtual try-on tool
Our virtual try-on uses your device’s camera to measure your PD while you preview frames. Follow the on-screen steps, keep your face straight to the camera, and make sure the room is well lit.
Is my PD measurement realistic?
Most adults have a PD between 50mm and 75mm, with an average of about 63mm, based on a widely cited review of the research by Dodgson (2004). If your reading falls well outside that range, measure again. A PD in the 40s is common in children, but unusual in adults.
Don’t use the average in place of your own measurement. It is a population average, not a safe default for your glasses.
How accurate does your PD need to be?
Aim to be within 1mm. Small errors matter more as your prescription gets stronger.
When the optical centre of a lens isn’t in front of your pupil, the lens bends light slightly sideways. This “prismatic effect” grows with both the size of the error and the strength of the lens. For example, a 2mm error with a −6.00 lens produces about three times the prismatic effect of the same error with a −2.00 lens.
If your glasses feel wrong, a PD error can show up as eye strain, headaches, or in stronger prescriptions a feeling that your eyes are pulling. If you have a high prescription (we define it as sphere ±4.00 or above, or cylinder ±2.00 or above), we recommend having your PD measured by an optician.
Single PD or dual PD: what’s the difference?
- Single (binocular) PD is one number, such as 63. It’s the total distance between your pupils.
- Dual (monocular) PD is two numbers, such as 31.5/31.5 or 31/32. Each measures from the centre of your nose to one pupil.
Many people’s faces are slightly asymmetric, so the two halves differ by a millimetre or so. Dual PD is more precise, and it matters most for varifocals, where each lens has a narrow corridor that must line up with each eye. Dual PD is hard to measure accurately at home, so ask an optician if you’re ordering varifocals.
Distance PD and near PD
The methods above give your distance PD, which is what most glasses use. When you read, your eyes turn inwards slightly, so your near PD is usually about 3mm narrower. If you’re ordering reading glasses only, let us know, so the lenses can be centred for close work.
Measuring a child’s PD
A child’s PD changes as their face grows, and small errors matter more for young eyes. We recommend having a child’s PD measured by an optician rather than at home.
Frequently Asked Questions
Is PD on my UK prescription?
Not usually. PD is a dispensing measurement, not part of the prescription. You can ask the practice that tested your eyes whether they have it on record.
Can I use my old glasses to find my PD?
Not at home. An optician can find where the optical centres are with a lens measuring instrument (a focimeter), but you can’t reliably judge this by eye.
Does my PD change?
Adult PD is generally stable. Children’s PD increases as they grow.
What if my two readings differ?
Take at least three readings and use the average. If they differ by more than 1–2mm, check your light, keep your head still, and try the friend method.
What happens if my PD is wrong?
A small error with a weak prescription often goes unnoticed. With a stronger prescription it can cause eye strain, headaches or blurred vision. If new glasses feel wrong, contact our team.
