Measuring my pupilary distance

I am in the process of ordering a pair of eyeglasses over the internet (I will use these glasses for about a year, until my cataracts are ready for removal, and I do not wish to spend lots if money going to a commercial vision care establishment).  I am under regular care of an opthalmologist for my history of torn retinas.  He has given me a prescription for lenses, but I do not see the necessary measurement of my pupilary distance.  My question is this:  How can I, perhaps with the help of another person and a metric ruler, measure this distance?  That is, what is the most accurate way to do this? Or----would my opthalmologist have a record of my PD in his file, as a matter of course?

[John C Hagan III, MD, FACS, FAAO]

Hello Dukeman. The pupillary distance (PD) is normally measured by the optician, that is the person that fills the ophthalmologists glasses prescription and makes the glasses. It is part of the service the optician performs and does not involve an extra expense.

JCH III MD Ophthalmologist