PD Measurement

How to Measure Your Pupillary Distance (PD) at Home: The Method Optometrists Use

Three methods to measure PD accurately, from ruler to app to online tool. Which one is reliable enough for ordering glasses?

Updated May 22, 2026 · 8 min read
Quick Answer: You can measure your PD (pupillary distance) at home with a millimeter ruler and a mirror. Stand 8 inches from the mirror, align the ruler's zero with your left pupil center, then read the measurement at your right pupil center. Most adults have a PD between 54–74mm. For the most accurate result, measure 3 times and average the numbers. PD measurement apps can also work, but the ruler method is equally reliable when done carefully.
Skip the details — here's what to do
  • The ruler method (with a friend) is accurate enough for most online orders — it takes 2 minutes
  • If your prescription is strong (above ±4.00), use an app or online tool for more precision
  • Single PD (one number like 63mm) is what most online retailers ask for
  • Don't guess your PD — wrong PD can cause eye strain and headaches

What Is Pupillary Distance (PD)?

Pupillary distance is the measurement in millimeters between the centers of your two pupils. It is essential for ordering prescription glasses online because it tells the optical lab exactly where to position the optical center of each lens.

There are two types of PD:

PD TypeWhat It MeasuresTypical Range
Single PDDistance between both pupil centers54–74 mm
Dual PDDistance from nose bridge to each pupil27–37 mm each

Method 1: Ruler and Mirror (Most Reliable)

This is the method most optometrists use in-office. You need a millimeter ruler (not inches).

  1. Stand in front of a mirror, about 8 inches away
  2. Hold the ruler against your brow, just above your eyes, with the millimeter side facing you
  3. Close your right eye. Align the ruler's zero mark with the center of your left pupil
  4. Open your right eye and close your left. Read the millimeter mark aligned with the center of your right pupil — this is your single PD
  5. Repeat 2–3 times and average the results

For dual PD: Measure from the center of each pupil to the center of your nose bridge. The two numbers should add up to your single PD.

Method 2: PD Measurement Apps

Several apps can measure PD using your phone's camera. Popular options include PDCheck, GlassesOn, and EyeMeasure.

Accuracy: Most apps claim ±1mm accuracy. Independent testing shows they are reasonably accurate for single PD but less reliable for dual PD measurements. For high prescriptions (above ±4.00), even 1mm of error can cause discomfort, so we recommend the ruler method as a cross-check.

Method 3: Online PD Measurement Tools

Many online glasses retailers offer web-based PD measurement tools that use your webcam or a photo with a credit card for scale.

Accuracy: These tools work well for moderate prescriptions but may have 1–2mm variance. They are sufficient for most single vision orders but consider the ruler method for progressive lenses where precision matters more.

Which Method Should You Use?

Your SituationRecommended Method
Single vision, low to moderate prescriptionAny of the three methods
Single vision, high prescription (±4.00+)Ruler method (measure 3x, average)
Progressive lensesRuler method + dual PD measurement
Large difference between eyesAsk optometrist to measure (asymmetric PD)

Common PD Measurement Mistakes

  • Looking at the ruler instead of straight ahead — your pupils shift when you look down. Focus on your reflection in the mirror.
  • Using an inches ruler — PD must be measured in millimeters. Converting from inches introduces rounding errors.
  • Measuring only once — always measure at least 3 times and average the results.
  • Confusing single and dual PD — Single PD is one number (e.g., 63mm). Dual PD is two numbers (e.g., 31.5/31.5). Make sure you enter the right format when ordering.

How PD Affects Your Glasses

If your PD is wrong by more than 2mm, you may experience:

  • Eye strain and headaches
  • Blurred vision despite correct prescription
  • Feeling "off" or dizzy when wearing glasses
  • Fatigue after short periods of wear

For progressive lenses, even a 1mm PD error can cause the reading zone to be misaligned, making the glasses uncomfortable or unusable.

Frequently Asked Questions

Yes, you can measure PD by yourself using a millimeter ruler and a mirror. Measure 3 times and average the results. The ruler-and-mirror method is the same technique optometrists use in their offices.
Most adults have a single PD between 54–74mm. The average PD for women is about 60–63mm and for men about 63–66mm. A PD outside this range is not unusual and does not indicate any problem.
Most online retailers accept either single or dual PD. If you only have your single PD, the retailer will typically divide it in half. However, if your eyes are not evenly spaced (which is common), dual PD gives a more accurate result, especially for progressive lenses.