“A person with 20/20 vision can see what an average individual can see on an eye chart when they are standing 20 feet away,” says Dr. McKinney. McKinney is an ophthalmologist and glaucoma specialist at Eye Health Northwest, Oregon City, Ore. An eye chart measures visual acuity, which is the clearness or sharpness of vision. The top number is your distance in feet from the chart. The bottom number is the distance at which a person with normal eyesight can read the same line. For example, if you have 20/30 vision, it means your vision is worse than average. Twenty feet away, you can read letters most people see from 30 feet.

Source: American Academy of Ophthalmology