Single PD vs dual PD: which one do you need?
Glasses order forms ask for PD in two formats, and the difference confuses almost everyone the first time.
Single PD (binocular)
One number: the distance between the centers of your two pupils, e.g. 63 mm. This is what most standard order forms want, and it is fine for ordinary single-vision glasses with mild prescriptions.
Dual PD (monocular)
Two numbers: the distance from each pupil to the center of your nose bridge, e.g. 31 / 32. The two rarely match exactly because faces are naturally asymmetric.
When dual PD actually matters
- Progressive and bifocal lenses - the reading corridor is narrow; a 1 mm offset is noticeable.
- Strong prescriptions - the prism effect of a misplaced optical center scales with lens power.
- Noticeably asymmetric faces - if your two values differ by 1.5 mm or more, dual PD keeps each lens centered.
Converting between them
Adding your two dual values gives your single PD exactly. Going the other way, splitting a single PD in half, is only an approximation; it assumes your nose sits exactly midway between your pupils, which is often not true.
The practical answer: measure once in a way that gives you both. PDgo reports total PD plus left-eye and right-eye values in the same 10-second measurement, so whichever format the store asks for, you have it. You can also measure manually with our free on-screen PD ruler, though reading per-eye values in a mirror takes patience.