Measure Your Pupillary Distance (PD) From a Photo

Pupillary distance (PD) is the distance between the centres of your two pupils, in millimetres. Glasses use it to put the optical centre of each lens right in front of each pupil. Most adults measure between about 54 and 74 mm, and the average is about 63 mm.

There are three numbers you may be asked for. Distance PD is measured with the eyes looking far away and is used for most glasses. Near PD is about 3 mm smaller, because the eyes turn inward to read, and is used for reading glasses. Single, or monocular, PD measures each eye from the centre of the nose bridge, written as two numbers such as 31.5/31.5; it matters most for progressive and strong lenses.

A photo has no scale of its own, so the tool needs an object of known size in the same picture. Bank, ID and most loyalty cards share one international size, ISO/IEC 7810 ID-1: 85.6 mm wide. The face landmark model that runs our other tools finds the centre of each pupil; you drag two lines onto the card's left and right edges; and the tool scales the distance between the pupils by the card's width.

Why a card and not the iris? Some tools use the width of the iris, which is about 11.7 mm in most adults, as the ruler. We tested that approach on 19 official NASA astronaut portraits: it read about 5 mm low on average and barely told wide-set eyes from narrow ones, so we do not use it.

The card sits on your forehead, a little closer to the camera than your eyes, and looking at a nearby camera turns the eyes slightly inward. Both make a photo read a little small, which is why the steps ask you to look far away and take the photo from a distance. Treat the result as a careful estimate: it is not a replacement for a measurement by an optician.

No card, or want to check the result? A millimetre ruler and a mirror work too:

• Stand about 20 cm (8 inches) from a mirror and hold the ruler flat against your brow, just above your eyes.
• Close your right eye and line up the ruler's zero with the centre of your left pupil.
• Close your left eye, open your right eye, and read the millimetre mark at the centre of your right pupil. That number is your distance PD.
• Repeat it three times and use the average. Having someone else measure you while you look at a distant object is more accurate than measuring yourself.

Online glasses stores ask for PD as one number, such as 63, or as two, such as 31.5/31.5. Use the distance PD for everyday and distance glasses and the near PD for reading glasses. For progressive lenses, strong prescriptions or prism, ask your optician to measure you, because small differences matter more there.

This tool is for adults. It gives an estimate for choosing and ordering glasses, not a medical measurement.