Frame Sizing Guide - Where Fit Meets Function
With Sightline lenses it's totally cool to stray a few millimeters off your PD if there is a must have style for you. Get in the ballpark, then find a frame style to suit your groove.
Got a smart phone? Get your PD with one of these two measuring tools.
Moving on ...
2.) Frame Size - Not all frames fit all folks. We group frames into three size groups; narrow, medium, and wide.
Getting you the correct frame width means your Sightline readers will be comfortable to wear, and the optics will be optimized.
Your guess is probably good enough. If you want to dig in, here is a tool for How to Measure Your Face for Glasses.