Re: Convert the visual acuity from Snellen chart to the # for my prescription?

Hi Haik, As a general rule, if you are nearsighted (myopia), for every line on the Snellen chart you cannot read, you are about -0.25 prescription. For example, 20/40 would be third line from 20/20 so you would be about - 0.75. Someone who can only see 20/200 is about -2.25. These are just approximations and varies from individual to individual. Also, this doesn't account for any astigmatism correction. For astigmatism, you are generally -0.50 for each line you cannot read. For example if you have a -2.00 of cylinder correction (astigmatism) you are likely seeing 20/50. Of course a lot of people are nearsighted and have astigmatism which complicates things. Hyperopic or farsighted people tend to have better vision based on presription. A +3.00 person (depending on age and other characteristics) might see as well as 20/40 uncorrected whereas a -3.00 person probably will be no better than 20/200 unless they squint of course! The snellen chart goes as follows starting from best visual acuity (ability to discern resolution): 20/20 20/25 20/30 20/40 20/50 20/60 20/70 20/80 20/100 20/200 20/400 I hope this helps and best of luck! Kenton L. McWilliams, O.D.