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How to measure your PD

PD, or pupillary distance, is the distance in millimetres between the centres of your pupils. It is the number that places the optical centre of each lens in front of each eye, so measure it carefully or ask your optician for it.

Woman facing the camera and holding the side of her glasses
Photo: Pavel Danilyuk / Pexels
Distance PD between pupil centres, separate right and left monocular values, and a smaller near PD when the eyes converge.Distance PD: pupil centre to pupil centreRight PDLeft PDNear PD is usually smaller: use measured values
Distance PD between pupil centres, separate right and left monocular values, and a smaller near PD when the eyes converge.

What PD measures

Pupillary distance is measured from the centre of one pupil to the centre of the other. Most adults measure between 54 and 74 mm, with an average a little above 60, and children usually between 43 and 58 mm. It is sometimes written on your prescription and sometimes kept on file by your optician, so ask them first.

Single and dual PD

Single PD is one total number, such as 63. Dual PD gives the right and left eye separately, measured from the centre of your nose, such as 32 and 31. The two sides are often slightly different. If you have both, use dual PD; it centres each lens more precisely.

How to measure with a ruler

  1. Stand about 20 cm in front of a mirror or, better, ask someone to face you at arm's length.
  2. Hold a millimetre ruler flat against your brow, just above your eyes.
  3. Look straight ahead at a point in the distance, not at the ruler.
  4. Line the zero up with the centre of your right pupil.
  5. Read the number at the centre of your left pupil.
  6. Repeat three times and use the average.

If a helper is measuring, they should close their right eye when reading your right pupil and their left eye when reading your left, so their own eyes do not shift the result.

Two ways to measure on your own

With a mirror: stand about 20 cm from a mirror and hold the ruler flat against your brow. Close your right eye and line the zero up with the centre of your left pupil. Then close your left eye, open your right and read the number at the centre of your right pupil. Keep your head still while you switch.

With a card for scale: take a straight-on photo of your face while holding a card the size of a credit card flat against your forehead, numbers turned away. A standard card is 85.6 mm long. Measure the card and the gap between your pupils on the photo in the same units, then scale the gap to millimetres. Take a second photo and compare; if the two differ by more than a millimetre, ask your optician.

Distance and near PD

Your eyes turn inward when you read, so near PD is smaller than distance PD, usually by about 3 mm. If you only have a distance PD and are ordering reading glasses, take 3 mm off; with dual PD, take 1.5 mm off each side. A near PD written on your prescription is always better than this estimate.

When a home measurement is not enough

Progressive lenses also need a fitting height, measured with the chosen frame on your face, and strong prescriptions are less forgiving of small errors. In those cases an optician's measurement is worth the visit.

Entering your PD

The lens step accepts one total PD or separate right and left values. Enter right and left in their labelled fields and never swap them. If your results vary by more than a millimetre, measure again or ask your optician. A PD outside 43 to 80 mm brings up a reminder to check it; you can still continue if it is right.

Common mistakes

  • Looking at the ruler instead of a point in the distance, which makes the reading smaller.
  • Measuring from the edge of the iris instead of the centre of the pupil.
  • Tilting the head, so one eye sits higher than the other.
  • Typing a near PD into a distance prescription.

Read your prescription

Where PD appears on your paperwork.

Progressive lenses

Why they need more than PD.

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PD questions

Is my PD on my prescription?

Sometimes. Many opticians record it separately; ask for it when you collect your prescription.

What happens if my PD is a little off?

Small errors of a millimetre are usually comfortable. Larger errors can cause eye strain, especially with strong prescriptions.

Can I use my PD from an old prescription?

Adult PD changes very little, so a recent measurement is usually fine. Children's PD changes as they grow.

Which PD should I use for reading glasses?

Near PD. If your prescription gives only a distance PD, use about 3 mm less, or 1.5 mm less on each side for dual PD.

Can I measure PD from a photo or a mirror?

Yes, both methods are described above. A helper with a ruler is the most accurate; if your results vary, ask your optician.

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