How to Read Contact Lens Prescription As a Snellen Vision

Updated April 17, 2017

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.

Things You'll Need

  • Copy of your contact lens prescription
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About the Author

Beth Richards, a freelance writer since 2002, writes about health and draws from her 25 years as a licensed dispensing optician. She has authored several books, writes for national magazines including "Country Living" and "Organic Family" and is a health and wellness features writer for several publications. She is earning a Bachelor of Arts in English from the University of Maryland.