How to measure pupillary distance (PD): 3 methods compared

Updated July 2026 · 5 min read

You're checking out on an online glasses store and it asks for your PD - pupillary distance, the distance between the centers of your pupils in millimeters. Your prescription probably doesn't include it. Here are the three practical ways to get it, from cheapest to most accurate.

Method 1: Millimeter ruler + mirror (free, fiddly)

1. Stand about 20 cm from a mirror, or ask a friend to help.
2. Hold a millimeter ruler flat against your brow, straight across both eyes.
3. Close your right eye and align the ruler's 0 with the center of your left pupil.
4. Without moving the ruler, close your left eye and read the mark at the center of your right pupil.
5. Repeat 3 times and take the most common value.

Accuracy: ±1–2 mm if you're careful. The hard part is keeping the ruler still and your head straight - a small tilt shifts the reading. Closing alternate eyes matters: it cancels the parallax error from looking at a mirror up close.

Method 2: On-screen PD ruler (free, no printer)

Same idea, but the ruler is on your phone or laptop screen, calibrated against a bank card (every standard card is exactly 85.60 mm wide). We built a free online PD ruler you can use right now - calibrate, hold it under your eyes in a mirror, read the value.

Accuracy: similar to a physical ruler, and easier since screen markings are crisp. Still manual, still needs a steady hand.

Method 3: iPhone 3D camera, automatic (fastest, most consistent)

iPhones with Face ID carry a TrueDepth camera - a 3D sensor that measures real distances, not flat pixels. That solves the fundamental problem of photo-based PD tools: a flat photo can't tell how far your face is, so it has to guess the scale. A depth camera doesn't guess.

PDgo uses it like this: you tap once, align your eyes with the on-screen guide, and it captures three rounds automatically in about 10 seconds. You get:

• Total PD, plus left-eye and right-eye PD (many faces are 1–2 mm asymmetric - dual PD is what precise labs use)
• A confidence rating, so you know whether to re-take
• A saved record for the next time you order

Everything is computed on the device; camera frames are never uploaded.

Which method should you use?

One last thing: PD is a fitting measurement, not a prescription. For eye health and prescription changes, see your optometrist.