Millimetre measurement
Pupillary Distance Measurement
PD is the distance between the centres of your pupils, and every online eyewear order asks for it. This measures it with your camera using the width of your own iris as the reference. Nothing is uploaded.
The measurement tool
How this measures Landmarks, formula, tolerance
- What is measured
- The straight-line distance between the two iris centre landmarks, measured flat rather than in three dimensions.
- Landmarks used
- Points 468 and 473, the iris centres. Points 469, 471, 474 and 476 give iris width. Points 33 and 263 give eye corners, used for the level check only.
- How pixels become millimetres
- Horizontal iris diameter is close to 11.7mm in adults, varying by about 0.5mm regardless of age, sex, or ethnicity, and unlike every other distance on a face it does not change with expression. Measuring it in pixels gives a millimetres-per-pixel figure for that frame, which converts every other distance.
- Where the tolerance comes from
- The 0.5mm spread in iris diameter divided by the 11.7mm reference is 4.3 percent, and that carries through to everything derived from it. It is a real limit of the method, not a safety margin.
- When a frame is refused
- Beyond 12 degrees of head rotation the shutter will not fire. A turned head foreshortens both the iris and the pupil gap, so the number would read small and look perfectly plausible. Beyond 15 degrees of tilt you are asked to level the phone.
- Why it takes a moment
- The result is the median of about 45 frames, not a single one. Landmarks jitter by a pixel or two on a handheld phone, and one frame bakes that jitter into the answer. Movement under 2 percent across the window is reported as a steady capture.
- Near PD
- Reading distance PD is shown as distance PD minus 3mm. That subtraction is the long-standing optical convention for the inward turn of the eyes at reading range, not a second measurement. It is labelled as a convention wherever it appears.
Every figure above is checkable against the measurement engine itself, which is plain JavaScript served with this page: fc-engine.js
Questions people ask
What is a normal pupillary distance?
Online eyewear retailers generally accept adult entries between about 54mm and 74mm, and most adults sit somewhere in the middle of that band. There is no correct PD. It is a dimension of your face in the same way your shoe size is a dimension of your foot, and the only thing it affects is where the optical centre of a lens needs to sit.
Can I measure PD myself accurately enough to buy glasses?
For single-vision glasses, usually yes. This tool reports to about plus or minus 4.3 percent, which is close enough that most single-vision prescriptions are unaffected. For progressive or varifocal lenses it is not, because those designs need each pupil placed within roughly a millimetre. Ask your optician for the measured figure in that case.
What is the difference between distance PD, near PD, and monocular PD?
Distance PD is the gap between your pupils when you look at something far away, and it is the figure most order forms want. Near PD is the same gap at reading range, which is smaller because your eyes turn inward slightly to focus close up. Monocular PD is each pupil measured separately from the centre of your nose bridge, which matters when the two halves of your face are not identical, and almost nobody is symmetrical.
Why does my PD change slightly each time I measure it?
Small differences between takes come from head angle, distance from the camera, and where your eyes were focused. This is why the result is a median across a short capture rather than a single snapshot, and why the tool reports how much the measurement moved while it was recording. A steady capture is a more trustworthy one, and the tool says which you got.
Is my photo or video uploaded anywhere?
No. The face detection model is downloaded to your browser and the camera frames are processed there. No image, no landmark, and no measurement is transmitted. You can confirm this by opening your browser network tab while the tool runs: after the model file loads, nothing further is sent.
Can I measure my PD with a ruler instead?
Yes, and the manual option on this page takes that measurement. Stand about arm's length from a mirror, hold a millimetre ruler against your brow, close your right eye, line the zero mark up with the centre of your left pupil, then switch eyes without moving the ruler and read the number at the centre of your right pupil. Doing it three times and taking the middle value removes most of the error.