How to Administer a Snellen Eye Test

1). Ensure that the Snellen eye chart is printed to the proper dimensions. The top line should consist only of only one letter. This letter should be 88 mm tall and the letters should become gradually smaller as you move through the lines.
2). Ensure that the room is well lit, but not in full sunlight, as this will distract from the viewer's normal eyesight. It is best to use overhead lights, as this will provide an even distribution of light across the room. Ensure that no distractions are present. Try to limit the amount of movement in the room.
3). Position the patient 20 feet away from the Snellen eye chart. Ask the patient to cover one eye with an index card. Have him begin by reading the top line of the eye chart. Then, have the patient move on to the line underneath and continue reading the letters until he can no longer make them out. Record the letters as the patient reads them.
4). Compare the patient's results against the actual letters on the chart. The last line she was able to see and read correctly determines her visual acuity. The first line is labeled as 20/400, which means that someone with average visual acuity can read this line from 400 feet away. The second line is 20/200, the third line is 20/100, fourth is 20/50, fifth is 20/40, sixth is 20/30 and the seventh line is 20/20.
5). Repeat the process on the other side to determine the visual acuity of the other eye.