PD on an eye prescription stands for pupillary distance, the space in millimeters between the centers of your two pupils. It tells the lab exactly where to place the optical center of each lens so you’re looking through the sharpest point. The average adult PD is about 63 mm, with most people falling between 50 mm and 70 mm.
What PD Looks Like on a Prescription
PD appears in one of two formats. A single number, like 63 mm, is called binocular PD. It’s simply the total distance from one pupil to the other. Two numbers separated by a slash, like 31/32 mm, is called monocular PD. This measures from the center of your nose to each pupil individually.
Monocular PD is more precise because most people’s faces aren’t perfectly symmetrical. Your right eye might sit slightly closer to or farther from your nose than your left. If you add the two monocular numbers together, you get the binocular PD (31 + 32 = 63). Monocular measurements are especially important for progressive lenses and bifocals, where each lens has distinct viewing zones that need to align precisely with each eye.
Distance PD vs. Near PD
Your eyes converge slightly when you focus on something close, so the distance between your pupils actually shrinks when you read. Some prescriptions list two PD values: one labeled “distance” (or “far”) and one labeled “near.” The near PD is typically about 3 mm less than the distance PD. If your distance PD is 63 mm, your near PD would be roughly 60 mm.
Distance PD is used for general-purpose glasses and distance-only lenses. Near PD matters for dedicated reading glasses. Progressive and bifocal lenses use the distance PD because the lab calculates the near portion’s offset during manufacturing. If your prescription only shows one PD number, it’s almost always the distance measurement.
Why PD Might Be Missing
If you’re staring at your prescription and can’t find a PD number, you’re not alone. Many prescriptions don’t include it. Under the FTC’s Eyeglass Rule, a prescription only needs to contain the information required by state law, and only some states mandate that PD be listed. Optometrists are required to give you your prescription so you can shop wherever you want, but PD is technically considered a fitting measurement rather than a refractive finding.
That said, the FTC encourages eye care providers to share PD if they’ve measured it. If yours isn’t on the prescription, you can call the office and ask. They likely recorded it during your exam and can read it to you over the phone or add it to a copy of your records.
How to Measure PD Yourself
If you can’t get PD from your provider, you can measure it at home with a millimeter ruler and a mirror. Stand 8 to 12 inches from a mirror at eye level, then hold the ruler horizontally across the bridge of your nose. Close your right eye and align the 0 mm mark with the center of your left pupil. Without moving the ruler, open your right eye and close your left. The millimeter mark lined up with the center of your right pupil is your binocular PD.
Repeat this three or four times and use the number that comes up most consistently. A few common mistakes throw people off:
- Looking at the ruler instead of your reflection. Keep your gaze fixed straight ahead in the mirror, not down at the numbers.
- Tilting your head. Even a slight tilt shifts the reading.
- Moving the ruler between steps. This is the most common cause of inaccurate results. Hold it completely still when you switch eyes.
For monocular PD, the process is similar. Instead of measuring from one pupil to the other, measure from the center of each pupil to the center of your nose bridge. You’ll end up with two separate numbers.
What Happens if PD Is Wrong
An incorrect PD shifts the optical center of the lens away from where your eye actually looks through it. For mild prescriptions, being off by a millimeter or two may not cause noticeable problems. But for stronger prescriptions, even a small error creates a prismatic effect, essentially bending light in the wrong direction. The result is eyestrain, headaches, blurry vision, or a sense that something feels “off” when you put on new glasses. If a new pair of glasses gives you persistent discomfort after a reasonable adjustment period of a few days, an incorrect PD is one of the first things worth checking.
Typical PD Ranges
Most adults measure between 50 mm and 70 mm, with 63 mm as the average. A small number of adults fall outside that window, ranging as low as 45 mm or as high as 80 mm. Children have smaller PD values that increase as their faces grow, which is why kids’ prescriptions should be remeasured at each visit rather than reusing an old number. If you measure yourself and land well outside the 50 to 70 range, it’s worth double-checking your technique or having a professional confirm the number before ordering glasses.