Simplest formula for converting mm to diopter or diopter to mm = 337.5 / r(mm) or 337.5 / D
Here remember an approximated corneal refracted index is used which equals 1.3375 instead of using the actual refractive index of the cornea which is equal to 1.376. As we know by now keratometer measures the anterior radius of curvature and gives us value in meters. But when dioptric power will be calculated using the actual refractive index of the cornea (1.376) it only shows anterior corneal surface refractive power, calculating only anterior corneal refractive power is baseless because we usually want to take the total power of the cornea which is a combination of anterior and posterior curvature of the cornea. That reason we use approximated or keratometric refractive index (1.3375) of the cornea which is equal to 90 % of anterior corneal refractive power and the left 10 % will be neutralized by posterior curvature of the corneal surface.