What is the formula to measure base curve of a contact lens?

• A. 337.5/diopters = mm
• B. 337.5*diopters = mm
• C. diopters = mm/337.5
• D. mm = 337.5 / Power

Answer: (A)

Base curve is the inner-surface radius of a contact lens, measured in millimeters, and it relates inversely to the lens power. In practice, a quick, standard conversion is base curve (mm) ≈ 337.5 divided by the lens power (diopters). This means higher diopter powers correspond to smaller base-curve radii, reflecting the steeper curvature needed to achieve that power. The constant 337.5 is an agreed-upon factor that makes the units workable in clinical approximations. Using a multiplication or a direct mm-to-D division would misrepresent the inverse relationship between curvature and refractive power. Remember, this is a handy estimate; actual fitting also depends on keratometry, corneal shape, and lens material.