Question 1

What is pupillary distance?

Accepted Answer

Pupillary distance (PD) is the distance between the centres of your pupils in millimetres. Lens makers use it to put the optical centre of each lens directly in front of each eye; if the centres are off, the lenses act as weak prisms and can cause eye strain or blur. It appears on a prescription as one number (binocular, e.g. 63) or two (monocular, e.g. 31.5/31.5), sometimes with a second, smaller near value for reading.

Question 2

How accurate is an online PD measurement from a photo?

Accepted Answer

This tool uses your iris as the ruler: the human iris is 11.7 mm across with very little variation between people, so the pixel width of your irises converts the pixel distance between your pupils into millimetres. Google's published validation of the same iris-reference method found a mean error of about 4%, which is roughly ±3 mm on a typical PD. That is accurate enough for ordering everyday single-vision glasses; for strong prescriptions, progressives or if two readings disagree, confirm with the printable ruler or your optician.

Question 3

What is a normal pupillary distance?

Accepted Answer

Adult PDs cluster around 63 mm, with men averaging about 64 mm and women about 62 mm. Almost all adults fall between 54 and 74 mm, and children between about 43 and 58 mm. A PD anywhere in that band is normal - it is a fitting measurement, not a feature to have more or less of.

Question 4

What is the difference between distance PD and near PD?

Accepted Answer

Distance PD is measured with your eyes looking far away; near PD is measured while reading, when the eyes turn inward. Near PD is conventionally about 3 mm smaller. Everyday glasses and most online orders use the distance PD; dedicated reading glasses use the near PD. This tool reports the distance value and an estimated near value.

Question 5

Why does my prescription show two PD numbers?

Accepted Answer

Two numbers around 30-35 are monocular PDs: the distance from the bridge of your nose to each pupil, right eye first. They are more precise than one binocular number because most faces are not symmetric, and progressive lenses in particular are cut to them. Add the two together to get the binocular PD. Two numbers like 63/60 are distance and near PD instead.

Question 6

What happens if my PD is wrong by a few millimetres?

Accepted Answer

Each millimetre of error shifts the optical centre and adds a small prism effect proportional to the lens power: at ±1.00 D a 2 mm error is barely noticeable, at ±5.00 D the same error can cause headaches, pulling sensations or double vision. That is why weak prescriptions tolerate an estimate and strong ones should be measured by an optician.

Question 7

Can I measure my PD at home with a ruler?

Accepted Answer

Yes. Stand about 20 cm from a mirror, hold a millimetre 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 and read the mark at the centre of your right pupil without moving the ruler. Repeat three times and average. Our printable ruler page has the scale and a bank-card outline to confirm your printer kept it at true size.

Question 8

Does pupillary distance change with age?

Accepted Answer

It grows through childhood as the skull widens and is stable from the late teens onward. Small differences between measurements (a millimetre or so) are measurement noise, not your face changing. Weight, hairstyle and glasses do not change it.

Question 9

Is my photo uploaded to measure my PD?

Accepted Answer

No. The 478-point face mesh, the iris detection and the millimetre conversion all run in your browser. The photo is not transmitted, stored or used for training unless you later choose a feature that explicitly asks for it.

Printable PD Ruler

How to Measure Your PD With the Ruler

Printable PD Ruler

How to Measure Your PD With the Ruler