How to Measure Your Pupillary Distance at Home
If you want to order prescription glasses online in the UK, there is one measurement you will need beyond your optician's prescription: your pupillary distance (PD). Without it, even a perfectly written prescription cannot produce lenses that are correctly aligned for your eyes.
The good news is that measuring your PD at home is straightforward, takes about two minutes, and requires nothing more than a ruler and a mirror. This guide walks you through every method — from the simple ruler technique to smartphone apps — and explains exactly what to do with the number once you have it.
What Is Pupillary Distance?
Pupillary distance (PD) is the measurement in millimetres between the centre of your right pupil and the centre of your left pupil. It is used by lens manufacturers to position the optical centre of each lens directly in front of your pupils — the point where light bends most accurately through the lens for your specific prescription.
If the optical centre of a lens is positioned incorrectly — even by a few millimetres — it introduces a form of prismatic distortion that can cause blurred vision, eye strain, headaches, and difficulty focusing. For lower prescriptions the effect is smaller; for stronger prescriptions, an incorrect PD is one of the most common causes of discomfort with new glasses.
This is why having an accurate PD is not a technicality — it is a genuinely important measurement that directly affects how well your glasses work.
Single PD vs Dual PD: What Is the Difference?
Your PD can be expressed in two ways. A single PD is one combined measurement for both eyes together (for example, 64mm). A dual PD gives a separate measurement for each eye individually (for example, 33mm right / 31mm left). Both are valid — but dual PD is more precise and is the preferred format for progressive lenses and stronger prescriptions.
Most people's faces are not perfectly symmetrical — the distance from the bridge of the nose to each pupil is often slightly different between left and right. A dual PD captures this asymmetry, meaning each lens can be positioned with greater precision. For everyday single vision lenses at low to moderate prescriptions, a single PD is perfectly adequate. For progressive lenses or prescriptions above ±4.00, a dual PD is the better choice.
Typical adult PD values range from approximately 54mm to 74mm for a single PD, with an average of around 62–64mm for women and 64–66mm for men. For dual PD, each eye measurement is typically between 28mm and 37mm.
How to Measure Your PD at Home: The Ruler Method
What You Need
- A millimetre ruler — a standard plastic ruler works perfectly; a free printable paper ruler is available from many eyewear websites if you do not have one
- A well-lit mirror — bathroom mirrors work well; avoid dim lighting as it makes pupil centres harder to see precisely
- A friend to help if possible — having someone else read the measurement reduces the margin of error, though the solo method works well too
Step-by-Step: Measuring Your Single PD
- Stand approximately 20cm from your mirror, at a comfortable and natural distance. Look straight ahead at your own reflection — do not tilt your head.
- Hold the millimetre ruler horizontally against your brow, just above your eyes. Keep it as level as possible, parallel to an imaginary line connecting your two pupils.
- Close your right eye and align the 0mm mark with the centre of your left pupil. Hold the ruler firmly in position — do not move it once it is aligned.
- Without moving the ruler, open your right eye and close your left eye. Read the measurement at the centre of your right pupil.
- That measurement is your single PD in millimetres. Write it down.
- Repeat the process three times and take the average of the three readings for the most accurate result. It is common to get slightly different readings each time — this is normal, and averaging removes the variation.
Step-by-Step: Measuring Your Dual PD
- Position yourself and the ruler exactly as described above — 20cm from a mirror, ruler held horizontally against your brow.
- Align the 0mm mark with the centre of your nose bridge — the midpoint between your two eyes.
- With both eyes open, read the measurement at the centre of your right pupil. This is your right eye PD (typically written as the first number, e.g., 33).
- Without moving the ruler, read the measurement at the centre of your left pupil. This is your left eye PD (typically written as the second number, e.g., 31).
- Your dual PD is written as right/left — for example, 33/31. Repeat three times and average each eye separately.
The Most Accurate Method: Use a PD Measurement App
For the most precise PD measurement — particularly important for progressive lenses or stronger prescriptions — a dedicated smartphone app is more accurate than the ruler method for most people.
PD measurement apps work by using your phone's camera in combination with a reference object of known size — most commonly a standard bank card (85.6mm × 54mm) held up to your forehead. The app uses the card dimensions to calibrate the scale of the image and calculate the distance between your pupils with good accuracy.
When using a PD app, keep the following in mind:
- Use a card of known standard size — a UK debit or credit card is ideal; the dimensions are internationally standardised
- Take the measurement in good natural light — avoid direct sunlight, which can cause the pupils to contract and make them harder to detect
- Hold the phone at eye level and look directly at the camera rather than the screen, if the app allows it
- Take three readings and average them, exactly as with the ruler method
- Most reputable apps provide both single and dual PD values — use the dual measurement if you are ordering progressive lenses
Can I Ask My UK Optician for My PD?
Yes — and this is always the most reliable option if precision matters. In the UK, you are legally entitled to a copy of your glasses prescription after an eye test, and you are equally entitled to request your PD measurement. Your optician is required to provide your prescription on request, free of charge.
The PD may or may not have been measured as part of your eye test — not all opticians measure it as a standard step, as it is primarily relevant when dispensing glasses in-store. If it was not recorded, your optician can measure it for you at the practice, though some may charge a small fee for this as a separate service.
If you are ordering progressive lenses or have a strong prescription, asking your optician to measure your PD professionally is the most reliable approach and is worth requesting even if there is a modest charge involved.
How Accurate Does My PD Need to Be?
| Prescription Type | PD Accuracy Required | Recommended Method |
|---|---|---|
| Low (up to ±2.00) | Within 2–3mm is generally fine | Ruler method or PD app |
| Moderate (±2.00 to ±4.00) | Within 1–2mm preferred | PD app or optician measurement |
| Strong (above ±4.00) | As precise as possible | Optician measurement strongly recommended |
| Progressive / Varifocal lenses | Dual PD required — high precision | Optician measurement strongly recommended |
| Single vision reading glasses | Within 2–3mm usually acceptable | Ruler method or PD app |
As a general rule: the stronger your prescription and the more complex your lenses, the more important a precise PD measurement becomes. For everyday single vision lenses at a low to moderate prescription, a careful home measurement is perfectly adequate. For progressive lenses, investing a few minutes (or a small fee) to get a professional measurement is well worth it.
Common Mistakes When Measuring PD at Home
- Tilting your head — even a slight tilt introduces error. Keep your head straight and level throughout the measurement
- Looking at the ruler rather than straight ahead — your gaze must remain straight forward at your reflection, not downward at the ruler, or your pupils will shift position
- Poor lighting — dim lighting makes pupil centres difficult to identify accurately; measure in good natural light or a well-lit bathroom
- Only measuring once — always take three readings and average them; a single measurement is more likely to contain error
- Using a paper ruler that hasn't been printed at 100% — if you print a paper ruler, make sure your printer scaling is set to 100% (not "fit to page"), otherwise the scale will be incorrect
- Measuring over contact lenses — contact lenses can slightly shift pupil position; measuring without them (or with glasses removed) gives a more accurate result
What to Do Once You Have Your PD
Once you have your PD measurement, ordering prescription glasses online is straightforward. At Urban Optics, you simply:
- Browse our full frames collection and choose your frames
- Select your lens type — single vision, progressive, blue light, or any other option
- Enter your prescription details from your optician's prescription slip
- Enter your PD — either as a single value or as separate right/left dual values
- Complete your order — we handle everything from there
If you have any questions about your prescription, your PD, or which lens type is right for you, our team is always happy to help before you place your order.
For a full explanation of every value on your glasses prescription, read our guide: How to Read Your Glasses Prescription: A Simple UK Guide.
Frequently Asked Questions
What is the average pupillary distance for adults in the UK?
The average single PD for adult women is approximately 62–64mm, and for adult men approximately 64–66mm. However, PD varies considerably between individuals and the only reliable way to know your specific measurement is to measure it directly. Do not estimate based on averages — even a few millimetres of error can cause visual discomfort.
Is measuring PD at home accurate enough to order glasses online?
For most single vision prescriptions at low to moderate strength, a carefully taken home measurement is accurate enough. The ruler method, when done correctly with three averaged readings, typically produces results within 1–2mm of a professional measurement — which is acceptable for most everyday prescriptions. For progressive lenses or strong prescriptions, a professional measurement is the safer choice.
Do I need a dual PD or a single PD to order glasses online?
Most online retailers, including Urban Optics, accept both. A single PD is fine for most single vision prescriptions. A dual PD is preferred for progressive lenses and higher prescriptions, as it accounts for any asymmetry between your left and right eye positions. If you have both values, provide the dual PD for the most accurate result.
My optician didn't include my PD on my prescription. Is that normal?
Yes — in the UK, opticians are not legally required to include PD on a glasses prescription as standard, since it is primarily used when dispensing lenses in the practice. However, you are entitled to request it, and your optician should either provide the measurement from your eye test records or offer to measure it for you. Some opticians charge a small fee for measuring PD as a separate service.
Can an incorrect PD cause headaches?
Yes. An incorrect PD means the optical centres of your lenses are not aligned with your pupils, which introduces a prismatic effect that forces your eye muscles to compensate. This can cause headaches, eye strain, blurred or doubled vision, and general visual fatigue — particularly with higher prescriptions. If you experience these symptoms with new glasses, an incorrectly measured PD is one of the first things to check.
Does PD change over time?
For most adults, PD is stable and does not change significantly over time. Children's PD increases as they grow. If you are an adult who has had their PD measured before, it is likely still accurate unless you have had any significant changes to your eye health. That said, it is always worth remeasuring when ordering a new pair of glasses if some time has passed since your last measurement.
Can I use the same PD for sunglasses as for regular glasses?
Yes — your PD is a fixed measurement of your eye spacing and applies to all prescription lenses regardless of frame type. The same PD value is used whether you are ordering prescription glasses, prescription sunglasses, or any other prescription eyewear.
Got your prescription and PD ready? Browse our full range of frames at Urban Optics and order your prescription glasses online — with free UK delivery on orders over £50 and free returns.
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