Contact lenses are a medical device that are prescribed based on individual needs. Your doctor determines the power and fit of the lenses based on your vision correction and shape of your eye. Equipment used by eye doctors include the Snellen Chart and phoropter for visual acuity and distance correction and a keratometer to determine the curves of your cornea. These help him determine your contact prescription and are written as a series of different numbers.
Read the prescription to determine right and left eye. OD is right eye and OS is left eye.
Find the power of your contact lens. This may be written as power or pwr.
Interpret the Snellen Chart, the test the doctor gives you with the letters on the wall. 20/20 vision means that you can read a certain size print when you are 20 feet away. If you measure at 20/40, that means at 20 feet from the chart you can read letters that a person with 20/20 vision could read from 40 feet away.
Determine the power of the lens as related to the Snellen Chart. 20/40 Snellen is approximately a power of 1.00, 20/60 to 20/100 can be power of 1.50 to 2.00, and 20/200 averages higher than 2.00 power.
Disregard other numbers on the prescription, such as base curve (B.C.) or diameter (dia.) as they are used to determine the fit of the lens, not your distance correction.
Snellen vision is only an estimate and can differ in relation to individual prescriptions. Other factors influence actual visual acuity, including astigmatism and other vision problems.