Thanks to e-how and Pamela Stephens
Refer to the eye glass prescription. Decoding the prescription is not as difficult as it might first appear. "Oculus dexter" (OD), "oculus sinister" (OS) and "oculus utrique" (OU) are Latin terms that mean right eye, left eye and both eyes. Sphere is the amount of correction needed in each eye. Cylinder is the amount of astigmatism.
Locate the sphere number and the cylinder number for the OD and OS. These numbers will likely be different for each eye.
Choose the numbers for one eye and calculate the diopter. Add the sphere number and half of the cylinder number to determine the diopter. For example, if the prescription indicates a sphere of 2.0 and a cylinder of -.50 this would be calculated as 2.0 - .25 = 1.75.
Round down when possible, because water is a natural magnifier. Diopters are in increments of .25. In the example, a diopter of 1.5 would be chosen.
Repeat the same formula to calculate the diopter for the other eye.
Purchase specific diopters for each lens in the scuba mask. Most full-service dive shops have step diopter lenses in stock and can easily replace regular lenses with them. You will not need to show your prescription to purchase step diopter lenses; however, dive shop professionals can double check your prescription to make sure that you purchase the correction that is best for your sight.