How to Measure Pupillary Distance at Home

You have a prescription and you want to order glasses online. The prescription has sphere, cylinder and axis but not pupillary distance, because in many places the optician isn’t required to include it.

PD is the distance between the centres of your pupils, and lenses are ground so their optical centres sit at that distance. Get it wrong and you’re looking through the wrong part of the lens.

How the measurement works

You need a reference of known width in the same photograph as your eyes, so the software can convert pixels to millimetres.

A standard payment card works anywhere in the world: the ISO/IEC 7810 ID-1 format specifies 85.60 mm wide, and every bank card, driving licence and ID card in that format matches it. Hold it flat against your forehead, in the plane of your face.

PD = (pupil separation in pixels ÷ card width in pixels) × 85.60 mm

Two things determine accuracy.

The card must be in the same plane as your eyes. Tilted forward or back, it appears narrower in the image, and the calculated PD comes out proportionally wrong.

You must focus on something distant. This is the error people don’t know about: eyes converge when looking at something close. Focusing on the camera at arm’s length pulls the pupils inward and measures a near PD, which is smaller than the distance PD most prescriptions use — typically by 3 to 4 mm.

Look at something across the room, past the camera, and have someone else take the photo if you can.

Single or dual PD

Single PD is the total distance between pupils. Dual PD measures each eye from the centre of the nose bridge separately.

Faces aren’t symmetrical. A single PD of 63 mm split evenly assumes 31.5 mm each side, and the actual split is frequently 32 and 31. For a mild single-vision prescription that’s usually tolerable; for progressives it isn’t.

If your optician provides dual PD, use it.

When home measurement isn’t good enough

Higher-power lenses have more prismatic effect away from the optical centre, so a 2 mm error matters far more than at low power. Progressives additionally need a fitting height, which cannot be measured from a photograph at all.

If you’re spending real money on progressive lenses, the cost of having PD measured properly is small against the cost of lenses you can’t wear comfortably.

Common mistakes to avoid

• Focusing on the camera instead of a distant point.
• Holding the reference card at an angle to your face.
• Measuring from a photo taken at very close range, where lens distortion affects the geometry.
• Using a single PD for progressive lenses.
• Measuring once and trusting it — take three photos and check the results agree within a millimetre or two.

How to do it with Pupillary Distance Measurer

The Pupillary Distance Measurer works from a photo with a known-width reference, in your browser.

1. Hold a standard card flat against your forehead, in the plane of your face.
2. Look at something distant, past the camera, not at the lens.
3. Take three photos and check the results agree.
4. For a high-power or progressive prescription, have it measured properly instead.

Other health tools are in the tools directory.

Frequently asked questions

How accurate is a home measurement?

Typically within a couple of millimetres if done carefully, which is acceptable for a mild single-vision prescription. Progressive and high-power lenses are far less forgiving.

Why look at a distance rather than the camera?

Because eyes converge when focusing close, which reduces the measured distance by several millimetres. Distance PD is what most prescriptions specify.

Should I use single or dual PD?

Dual, if your optician provides it. Faces are rarely symmetrical about the nose bridge, and a single PD split evenly assumes a symmetry that often isn’t there.

Final thought

Look past the camera, not at it. That one instruction accounts for most of the error in home PD measurement, and it costs nothing to get right.

Try the free Pupillary Distance Measurer