There are a multitude of criteria that factor into determining the proper size of glasses to purchase. Glasses are like clothes, Size 'X' for Brand 'A' often does not offer the same fit as Size 'X' for Brand 'B', so matching a singular measurement specfication is often not enough ot guarantee proper sizing. Your prescription should factor into the shape/size of the frame you choose. For example, high-minus (nearsighted) prescriptions are not typically suitable for large, circular frames. For those with multifocal prescriptions, adequate lens depth will be needed to provide multiple focal zones prescribed. Choosing a suitably sized frame does not guarantee a proper fit. Fit at the bridge, temples, around the ears, and the frame depth all factor into selecting a properly fitted frame. It is more than just matching numbers. Different styles of frames need to have different combination of measurement specifications to offer similar fit. Different frame designs and frame materials offer differing amounts of adjustability. Adjustments are often needed to obtain the best fit and comfort.