How to Measure Your Pupillary Distance (PD) at Home

What pupillary distance is, and why it matters

Pupillary distance is one number: the gap, in millimeters, between the centers of your two pupils. Opticians use it to position the optical center of each lens exactly where your eye looks through it. Get it wrong by a few millimeters and you introduce a small prism effect — the kind of thing that produces eye strain, headaches, or that vague “these glasses feel off” sensation, especially with stronger prescriptions.

When you buy glasses in a store, the optician measures your PD as part of the fitting. When you buy online — where most of the savings are — the retailer asks you for it, and a surprising number of prescriptions don’t include it. So you measure it yourself. Here’s how to do that accurately.

Method 1: Ruler and a mirror

This is the classic do-it-yourself method. You need a ruler with millimeter markings.

1. Stand about 20 cm (8 inches) away from a mirror.
2. Hold the ruler flat against your brow, just above your eyes, with the millimeter side down.
3. Close your right eye. Slide the ruler so the 0 lines up with the center of your left pupil.
4. Hold the ruler perfectly still, then close your left eye and open your right.
5. Read the millimeter mark that sits over the center of your right pupil. That number is your PD.

Look straight ahead the whole time and keep the ruler level — tilting it or glancing sideways is the most common source of error. Measure three times and take the average; if your readings vary by more than a millimeter or two, your alignment is drifting.

Method 2: Ask a friend

A second person removes the awkward one-eye-at-a-time dance and usually gives a steadier result.

1. Sit down and look at something far away — across the room or out a window, at least 3 meters (10 feet) off. This relaxes your eyes into their “distance” position.
2. Have your friend hold the millimeter ruler across the bridge of your nose, resting on your cheeks.
3. Keeping your gaze fixed on the distant object, they read the distance from the center of one pupil to the center of the other.

Because you’re staring into the distance, this gives you your far PD — the one you want for everyday glasses.

Method 3: Your iPhone (the easy, repeatable way)

Both ruler methods fight the same enemy: parallax. The moment your line of sight isn’t perfectly perpendicular to the ruler, the reading shifts. That’s why two careful people often get two different numbers.

An iPhone sidesteps the problem entirely. The front camera and TrueDepth sensor build a 3D map of your face and measure the straight-line distance between your pupils — no ruler to keep level, no eye to close, no friend to recruit. You hold the phone up, and the Pupillary Distance app reads your PD to within about a millimeter, the same tolerance opticians work to. It also gives you your near PD and your left/right monocular PD in the same pass, which the ruler methods can’t do easily. If you’d rather not download anything first, here’s how measuring your PD online with your phone camera works step by step.

Far PD vs. near PD

• Far (distance) PD is measured with your eyes relaxed on a distant target. It’s used for distance and everyday glasses.
• Near PD is measured with your eyes converged on something close, like a book. Because your eyes turn slightly inward to focus up close, near PD is usually about 3 mm smaller than far PD. It’s used for reading glasses.

If you only have one number and you’re ordering single-vision distance glasses, use your far PD. Progressive lenses use both.

Monocular PD (left and right)

A single PD is the total pupil-to-pupil distance. Monocular PD breaks that into two measurements — from the center of your nose bridge to each pupil — because almost no one’s face is perfectly symmetrical. Most single-vision orders are fine with a single PD, but for progressive lenses or strong prescriptions, two monocular values give a noticeably better fit. Measuring monocular PD with a ruler is fiddly; a depth-camera measurement handles it automatically.

Common mistakes (and how to avoid them)

• Parallax / looking sideways. Keep your gaze straight ahead and the ruler level. This is the #1 error.
• Measuring while your eyes converge. For far PD, fix your eyes on something genuinely far away — not the mirror an arm’s length in front of you.
• One reading. Always measure two or three times and average them.
• A worn or inch-only ruler. Use a clean millimeter scale; converting inches introduces rounding error.

So which PD number do you actually need?

For most people ordering single-vision glasses online: one far PD between roughly 54 and 74 mm. For reading glasses, your near PD. For progressives or a strong prescription, two monocular PDs. Whichever you need, measure it carefully — or let your iPhone measure all of them at once and skip the guesswork.