The Disabled World website makes available multiple versions of printable Snellen eye charts. This is the type of chart used in ophthalmologists' offices, and it can also be used at home to test visual acuity, as Disabled World points out.
Using the Snellen chart, normal vision is characterized as 20/20, states Disabled World. Someone with 20/20 vision is able to see at a distance of 20 feet what other people with normal vision see at that distance. To measure visual acuity, the chart must be read from 20 feet away with one hand over the eye not being tested. The lowest readable line determines the visual acuity of the person taking the test.