Pupillary distance (PD) measurement

Built by Nitin Raja · Last updated October 24, 2026

How to measure your pupillary distance

Your pupillary distance, or PD, is the distance between the centres of your two pupils in millimetres. Every pair of prescription glasses is made with it: the lab lines up the optical centre of each lens with each pupil. When the PD is wrong the lenses still have the right power, but you look through them slightly off centre, which can cause eye strain, headaches or a "swimming" feeling, especially with stronger prescriptions.

Shops ask for it when you order glasses online, and it is often missing from the prescription you got after your eye test. That is what this tool is for.

How this tool measures your PD

When you start, the page opens your front camera and runs a face-tracking model right in your browser. It finds the centre of each pupil and the outline of each iris (the coloured ring around the pupil). The iris is a handy ruler: in adults it is about 11.7 mm across, and it varies by only about half a millimetre from person to person. By comparing the distance between your pupils with the size of your iris, the tool works out your PD in millimetres without needing a ruler or a card.

It takes a reading from many video frames and keeps only the ones where you face the camera straight on, then uses the middle value so a blink or a small head turn does not throw it off. Because your eyes turn inwards slightly to look at a screen at arm's length, the camera sees a PD about 1.5 mm narrower than when you look into the distance, so the tool adds that back. The result is your distance PD, the number glasses are ordered with.

For a tighter result, use card mode. Hold any standard bank or ID card flat against your forehead and take a photo. Those cards are exactly 85.6 mm wide, so once you drag the two lines onto the card's edges, the tool has an exact scale for your face.

Measure PD online with your phone or webcam

You can measure PD online for free with whatever camera you have. On a phone, the front camera does the job; on a laptop or desktop, the webcam does. There is no app to install and no account: the page runs in an up-to-date browser, and the video is processed on your device, never uploaded. A phone held at arm's length usually gives the sharpest view of the iris, so use it if your webcam is low resolution or the room is dim. Card mode works the same way on both.

Getting an accurate reading

• Take off your glasses. Lenses bend light and hide the iris edge.
• Use even, bright light from in front of you, not behind you, so your eyes are not in shadow.
• Hold the screen at arm's length and level with your eyes, and look straight at it. The tool tells you if your face is turned or tilted.
• Measure two or three times. If the results agree within a millimetre, you can trust them.

Measuring PD with a ruler and a mirror

If you prefer the old way, stand about 20 cm (8 in) from a mirror and hold a millimetre ruler flat against your brow. Close your right eye and line up the 0 mark with the centre of your left pupil. Then close your left eye and open your right one, and read the mark under the centre of your right pupil. That number is your PD. Repeat it three times and take the average. A friend can do the same while you look at a point about 3 metres away, which gives a slightly more accurate distance PD.

What the numbers mean

Distance PD is the main number, used for everyday glasses, driving glasses and sunglasses. Near PD is for glasses you only wear to read or use a computer up close. Your eyes converge when you look at something near, so near PD is usually about 3 mm smaller. Single (monocular) PD splits the total into the distance from the middle of your nose to each pupil, such as 31.5/32. Few faces are perfectly symmetrical, so varifocals and strong lenses are better made with single PD. If a shop asks for one number, give your distance PD.

Average pupillary distance

The average adult PD is about 63 mm. In a 2004 study of nearly 4,000 adults, most people were between about 54 and 74 mm, and men averaged slightly wider than women (about 64.7 mm against 62.3 mm). Children's PD is smaller and grows as the face grows, so always measure a child again when they need new glasses rather than reusing an old number.

When to ask an optician instead

A phone measurement is fine for most single-vision glasses and sunglasses. For varifocals and bifocals, strong prescriptions (above about ±4.00) or a child's first glasses, a fitting by an optician is worth it, because those lenses are less forgiving of a millimetre or two. Many opticians will also simply tell you the PD they measured at your last eye test if you ask.

This tool helps you order glasses. It does not test your eyesight. Keep having regular eye tests, and see an optometrist if your vision changes.

Why we built this PD tool

Before building CheckMyHeight, I tested a lot of height and sizing websites to see what people need and what gets in the way. Pupillary distance is a common place to get stuck: online glasses shops ask for it, the prescription slip from an eye test often does not include it, and the usual home method needs a millimetre ruler held against your brow in a mirror, which is fiddly to read on your own.

So we built one that needs nothing by default, runs on the camera you already have, keeps the image on your device, and shows how it got the number.

What this PD tool measures

It measures the distance between the centres of your pupils from your front camera or webcam. Exactly what the code does:

• Finds your eyes with Google's MediaPipe Face Landmarker, running in your browser: the pupil centres and four points on each iris outline.
• Scales by the iris. The iris is taken as 11.7 mm across (white-to-white diameter 11.71 ± 0.42 mm in Rüfer et al., 2005), so pixels become millimetres without a ruler.
• Keeps only good frames: facing the camera straight on (checked from the nose tip and the outer eye corners), head tilted less than 7 degrees, both irises about the same size, at a usable distance. It collects 45 such frames and takes the median.
• Distance PD = the measured value + 1.5 mm, because your eyes turn in slightly to look at a screen at arm's length. Near PD = distance PD - 3 mm.
• Single PD: the pupil line is split where it crosses the bridge of the nose.
• Card mode (optional): a bank or ID card on your forehead, 85.6 mm wide (ISO/IEC 7810 ID-1), replaces the iris as the scale after you mark its two edges.

The constants were last checked on 24 October 2026. What it does not measure: your prescription, eyesight or eye health, the segment or fitting height that varifocals need, or vertex distance. It is a tool for ordering glasses, not an eye test.

Why this PD tool is different

• No props by default. The iris is the ruler; the card is an option for a tighter result, not a requirement.
• Your face stays on your device. The video is analysed in the browser and is never uploaded or saved.
• Three numbers, not one: distance PD, near PD and single PD for each eye.
• The method is shown, including the corrections it applies and the published iris size it relies on.
• No app and no account. It works with a phone camera or a laptop webcam in an up-to-date browser.
• It tells you when to see an optician instead: varifocals, strong prescriptions and a child's first glasses.

Measuring more of yourself? Try the height measurement or measure objects with your phone.