The base curve of a contact lens is the measurement of the back curvature of the contact lens, also known as the "fitting curve". It is the term used to describe the steepness of the lens needed to match the shape of the persons cornea. The base curve is a required part of any valid contact lens prescription. The base curve determines what type of fit is required for the lens to meet the curve of your eye. The base curve is usually written in millimeters or sometimes with the words: flat, median or steep. The base curve number represents how curved the contact lens is. This number is typically between 8.3 and 9 millimeters in soft contact lenses. If the base curve value is not appropriate for your cornea, even modest changes can result in discomfort and poor vision. The larger the base curve number, the flatter the lens is. That means a person with a less curved cornea would wear a contact lens with a higher base curve number; someone with a more curved shaped cornea would need a steeper base curve that has a lower number.