What is the equation for converting a contact lens base curve in diopters to a radius of curvature in millimeters?

• A. 337.5/x
• B. x/337.5
• C. 337.5*x
• D. 1000/x

Answer: (A)

When you convert a contact lens base curve from diopters to a radius in millimeters, you use a fixed conversion that links curvature to optical power. The practical relationship is radius (mm) = 337.5 divided by the diopter value. That 337.5 comes from combining the refractive index differences at the air–tear film interface with unit conversions, giving a convenient rule of thumb used in contact lens practice: a steeper curve (higher diopters) has a smaller radius, and vice versa.

So, if the base curve is given as x diopters, the radius in millimeters is 337.5 / x. For example, a 40 D base curve gives about 8.44 mm. The other formulas don’t match this established link between diopters and radius: they either invert the ratio, multiply where you should divide, or rely on a simpler 1/R relationship that doesn’t reflect how base-curve power is defined.