BC stands for Base Curve, which is the measurement of the back curvature of the contact lens. It is an important part of your contact lens prescription, and your prescription will be deemed “invalid” without it. The back curve has to be determined to appropriately fit the front curve of the cornea so that the contact lens fits comfortably. BC values range from about 8.0 to 10.0, and the doctor fits the lens with the curvature most appropriate for your eye. Most lenses come in several different BC values. If your prescription does not contain a BC value, it means that your brand only comes in one base curve.