Pupillary distance.

The short answer

Monocular PD is half the binocular PD when your face is symmetric: 63 mm binocular is 31.5 mm for each eye. Two monocular numbers add up to the binocular PD.

Worked example

Three ruler readings of 63, 64 and 63.5 mm average 63.50 mm, which rounds to 63.5 mm (nearest 0.5 mm, the way shops quote it). Split evenly that is 31.75 mm per eye. Going the other way, monocular 31.5 and 32 mm add up to 63.5 mm binocular.

How it works

Pupillary distance is the distance between the centres of your pupils, in millimetres. Binocular PD is the pupil-to-pupil total; monocular PD is each pupil to the centre of the nose bridge. The two monocular PDs always sum to the binocular PD, but they can differ by a millimetre or more because faces are not perfectly symmetric.

The equal split offered here is an assumption that holds for a symmetric face. If your optician gave two monocular numbers, use those. For reference, the 2012 US Army anthropometric survey (ANSUR II) reported an adult mean interpupillary distance of 61.7 mm for women (range 51.0 to 74.5) and 64.0 mm for men (range 53.0 to 77.0); the page warns when a result is outside 51 to 77 mm.

Measuring at home. A ruler reading is only an estimate and can be off by a millimetre or two. For multifocal or progressive lenses, or a strong prescription, have the PD measured by an optician. This tool does not measure from a photo.

Sources (opened 2026-10-08): Wikipedia, Pupillary distance (US Army ANSUR II table).

FAQ

How do I convert binocular PD to monocular PD?

Divide it by two if your face is roughly symmetric. A binocular PD of 63 mm gives 31.5 mm for each eye. If your optician measured each eye separately, use those numbers instead.

What is a normal pupillary distance?

In the 2012 US Army anthropometric survey, adult women averaged 61.7 mm and men 64.0 mm, with individual values from about 51 to 77 mm. Yours can be anywhere in that range.

Can two monocular PDs be different?

Yes. They can differ by a millimetre or more, and they always add up to the binocular PD.

How accurate is a ruler measurement?

It is an estimate and can be a millimetre or two off, which is why this tool averages several readings and rounds to 0.5 mm. An optician measurement is more reliable, especially for progressive lenses.

Why does glasses PD matter?

The optical centre of each lens should sit in front of the pupil. A mismatch adds unwanted prism, which matters more with stronger prescriptions.

Does this measure PD from a photo?

No. It only does arithmetic on numbers you enter. Nothing is uploaded.