How to Check Whether Your PD Measurement Is Right

Last updated: October 2026 | Reading time: 7 minutes

Short answer: no single reading proves a PD is right. You get confidence from agreement: several photos that match each other, a number inside the normal adult range, left and right values that make sense, and ideally a second method or a professional reading that lands close by.

Check 1: repeat it three times

Take at least three photos, move the phone away and back between them. If the results agree within about 1 mm, the random part of the error is small. If they scatter by 2 mm or more, something is changing between photos: your distance, head angle, card position or lighting. See why your PD changes between photos.

Agreement does not catch a consistent mistake, such as a tilted card every time. That is why the other checks matter.

Check 2: is it in a believable range?

Adult PDs commonly cluster around the low 60s in millimetres, and results far from that deserve a second look. Our tool rejects values outside a wide plausibility window, but anything unusual inside it is worth rechecking. For typical values by age and sex, see average PD by age and gender. Children's PDs are smaller (PD for children).

Check 3: do left and right look reasonable?

The two monocular values should add up to the total, and most people are within a millimetre or so of symmetric. A large gap such as 4 mm usually means a head turn or a card that is off-centre, not your face. Retake with the card centred and your head square to the camera.

Check 4: compare it with the old prescription or glasses

If you have a PD on an older prescription, or your current glasses fit well, compare. Opticians can also read the optical-centre distance on glasses you already own with a lensmeter, which is a quick way to check a measurement. PD can change slightly over a lifetime, see does PD change with age?.

Check 5: compare with a different method

Measure again with the mirror-and-ruler method (how to measure PD at home). Two different methods agreeing within a millimetre or two is reassuring; a big gap tells you at least one is off.

Check 6: look at the reported error

Our tool shows an estimated error in millimetres. If it says ±1 mm, treat the displayed number as a range. For a strong prescription, see how much PD error is too much.

When to get a professional measurement

• Progressive, bifocal or strong lenses
• Children's glasses
• Any measurement that you cannot reproduce
• Headaches or blur after trying glasses made from a self-measured PD (PD and eye strain)

Related

Common PD measurement mistakes · PD tool fixes · Try the tool