Smart Buys

Know Your PD

The number nobody tells you, and why it matters.

By James Stone

Issue #1 · Published Oct 19, 2026

Pupillary distance — PD — is the distance in millimeters between your pupils. It's the single most important number in your glasses after the prescription itself, because it tells the lab where to put the optical center of each lens. Get it wrong and you're looking through the wrong part of the lens: eyestrain, headaches, blur at the edges. It's that important.

Here's the scandal: many prescribers don't volunteer it. The PD is measured during your exam, written in the file, and then — in a lot of practices — not handed to you. Why? Because a PD in your pocket is a PD you can take to an online competitor. It's the one number that makes buying glasses anywhere possible, so gatekeeping it is, functionally, customer retention.

Your eyes. Your number. Your business. Ask for your PD.

Your prescription is yours by federal law — the FTC's Eyeglass Rule requires it to be handed over after a refractive exam, no purchase required. PD is handled separately, so ask for it directly: "Please include my PD on the prescription." If they resist, measure it yourself. Digital PD measurement via a photo is convenient and fine for most single-vision orders — but a professional measurement is still the most accurate — and there are free tools and apps that walk you through it with a credit card held to your forehead as a scale reference. For progressives or strong prescriptions, get it measured professionally; the tolerance is tighter.

Write it down. Save it in your phone. It's yours. Your eyes, your number, your business.

While you're at it, learn the other numbers on the temple: lens width, bridge width, temple length. Together with PD, they're the complete fit fingerprint. Any seller — online or in-store — can work with those four numbers. Memorize them like a phone number you actually call.

Sources: Cleveland Clinic, Pupillary Distance (PD): What It Is & How To Measure.