Base Curve (BC) is one of the main parameters of a contact lens and describes - in general - the shape of the contact lens measuring the radius of the sphere of the back side. The range usually goes from 8mm to 10mm. The Diameter (DIA) describes the size of the lens. During the fitting process your eye doctor will determine the base curve and the diameter that is right for you as it will ensure maximum wearing comfort and optimized vision correction.