How to measure your PD

A single number, in millimetres, usually between 54 mm and 74 mm for adults. It is your measurement, not the frame's. Once you know it, you can re-use it for every future pair of prescription glasses.

Single PD is one number (for example 62) that describes the full pupil-to-pupil distance. This is what most online stores ask for, and it is enough for single-vision lenses up to a moderate prescription.

Dual PD is two numbers (for example 31 / 31) — the distance from each pupil to the centre of your nose. Choose dual PD if you have a strong prescription, if you are ordering progressive lenses, or if your two eyes sit asymmetrically.

1. Stand 20 cm (about 8 inches) in front of a mirror, looking straight ahead.

2. Hold a millimetre ruler flat against your eyebrows.

3. Close your right eye. Line up the 0 mark with the centre of your left pupil.

4. Without moving the ruler, open your right eye and close your left. Read the millimetre line that lines up with the centre of your right pupil.

5. That number is your PD. Repeat 2–3 times and use the most consistent reading.

Ask a friend to help if you can — looking straight ahead while a partner measures gives the most accurate result.

If you can't measure cleanly or you don't trust the number, tick the I don't know my PD option in the lens flow. Our prescription review team will contact you and use a standard PD that matches your frame size for low/moderate prescriptions. For stronger prescriptions we will ask you to confirm by photo or with your optometrist before cutting lenses.

Contact support if your prescription is stronger than ±4.00, if you are ordering progressives, or if your prescription form lists your PD as a single number you don't understand. We would rather pause the order than ship a pair you can't see through.

If the optical centre of each lens does not line up with your pupil, every line you look at will be slightly bent. For weak prescriptions this is a mild annoyance. For stronger ones it causes eye strain, headaches, and double vision. PD is the single cheapest insurance against a bad pair of glasses.