Contact prescriptions differ from glasses prescriptions. Because contact lens prescriptions require different information, including the size and shape of the lenses, the two prescriptions contain different details. It is important to understand the differences before converting contact prescriptions to glasses.
Note the prescription power of the contact lenses. The power, also called diopter, is written as plus for farsightedness, such as +2.00, and minus for nearsightedness, or -2.00. Your power can start at 0.00 and increases in .25 steps to as high as 20.00 diopters or more. If your contact prescription is plus or minus 4.00 diopters or more, optical math is used to convert the prescription.
Check with your eye doctor to compensate for vertex distance if the prescription is stronger than plus or minus 4.00 diopter. The power changes depending on how far a corrective lens is from the eye. Contact lenses sit right on the cornea. Glasses lenses rest an average of 10 millimeters away. The lens power is different as you move a lens further from the eye and the numbers will be different in a contact and glasses prescription beginning around 4.00.
Measure the PD (pupil distance). Hold a ruler in front of the person to be measured and have them look straight ahead. Line up the 0 on the ruler in front of his right eye.
Look at the ruler as he continues to look straight ahead and measure where the lines of the ruler fall in front of the other pupil. From center of pupil to center of pupil is called the PD and averages 56 to 68 millimeters. This number is needed to determine where to put the optical center of the glasses lenses.
Do not try to convert toric power, the correction for astigmatism written as cylinder, from a toric contact lens prescription as this will be different for a spectacle prescription. If you have a considerable amount of astigmatism, the only way to obtain the spectacle powers is to refer to the original refraction or redo the eye exam.
Disregard the base curve and diameter of the contact lens prescription. These numbers are needed only for the fit of the contact lens and do not need to be considered for the spectacle prescription.