I had the optometrist write it down. I needed replacement lenses for my good glasses (broke one) and the best deal (by far) was Costco. The guy ignored the opt

I had the optometrist write it down. I needed replacement lenses for my good glasses (broke one) and the best deal (by far) was Costco. The guy ignored the optometrist's PD and measured it himself with a gadget he had and the number was quite different. The glasses came out OK though they don't guarantee satisfaction putting new lenses in an old frame they don't have data for. For ordering with Zenni, I tried the binocular method and then, I think averaged that with the other two numbers. The Zenni glasses are also fine. Sensitivity to centering may be greater if you have strong cylindrical. In my case it's less than 1/4 the spherical. The $20 Zenni frames aren't quite as light or nearly indestructible as the $400 frames of my 22 year old good glasses. Plus you have to bend the little nose pad things yourself to get a comfortable fit. But now that I wear Zenni glasses around the house all the time, I've gotten used to them, and I may have bought my last pair of expensive glasses ever. Don't mail order frameless or bifocal glasses though.