PD ruler (pupillary distance)

The distance between the centres of your two pupils, in millimetres. Glasses are made so that the optical centre of each lens sits in front of each pupil, so an online glasses order asks for it. Most adults’ PD lies between about 50 and 75 mm, around 63 mm on average (Dodgson, 2004); children’s are smaller.

Three ways, from most to least reliable: with a helper and the printed ruler, with a mirror and the printed ruler, or from a photo with a bank card in it (above). In each case measure three times and use the value that comes up most often. Print the ruler at Actual size and check it with a bank card first — a ruler printed at 97% reads a 63 mm PD as about 65.

Single PD is the whole distance from pupil to pupil. Dual or monocular PD gives each eye separately, measured from the centre of the nose bridge — for example 31.5/32. Faces are rarely perfectly symmetric, so for strong prescriptions and varifocals the two halves matter. The centre-notch ruler on the printable page reads each half; the two add up to the single PD.

When you read, your eyes turn inwards to look at the page, so the lines of sight cross the lenses of reading glasses closer together. Single-vision reading glasses are therefore made to a near PD, usually 3 to 4 mm less than the distance PD: for a 63 mm PD and reading at 40 cm it works out at about 59 mm. Opticians calculate it as PD × W ÷ (W + 27 mm), W being the reading distance.

Usually to within 1–2 mm when the card is flat, level and at the same depth as the eyes, and the camera is straight in front of the face. The card sets the scale, so a card held 1 cm nearer the camera than your eyes, with the camera 40 cm away, makes the result about 2.5% small — roughly 1.6 mm. A click one pixel off matters far less; the page shows how much. The photo is never uploaded: it is read and measured in your browser.

Because eyes turn inwards to look at something near. Looking into a phone held at arm’s length, each pupil moves about 0.8 mm towards the nose, so a selfie measures about 1.6 mm short. The page corrects for that from the distance you choose; looking at something across the room, past the camera, needs no correction at all.

The ruler reads any PD from 0 to 100 mm, but children find it hard to keep still and look at the right place, and their glasses are usually fitted in person. Ask the optician or optometrist for a child’s PD.

No. It is not a medical device. A PD you measure at home is a reasonable guide for ordering glasses online, but your optician’s or optometrist’s measurement is the authoritative one, and for varifocal or strong lenses many sellers recommend having it measured professionally.