What does "20/20" mean?

The top number in the Snellen fraction is always 20. It refers to the standardized visual acuity testing distance of 20 feet. The bottom number of the fraction describes the farthest distance from which a person with normal eyesight can recognize the smallest letters you can see when you are 20 feet from the chart.



Therefore, "20/20" means the smallest letters you can recognize at a distance of 20 feet are the same size as those a person with normal eyesight can see at the same distance. You have normal (20/20) eyesight.



But if you have 20/40 visual acuity, the smallest letters you can recognize at 20 feet a person with normal eyesight can recognize when they are 40 feet away. In other words, your visual acuity is only half as clear as that of a person with normal eyesight (20/40 = 50%).



The larger the bottom number to the Snellen fraction, the worse your eyesight is. For example, if you have 20/200 visual acuity, the smallest letters you can recognize at 20 feet a person with normal eyesight can recognize when they are 200 feet away. Your visual acuity is only 10 percent the level of normal eyesight (20/200 = 10%).



It's possible to have eyesight that is better than normal. In this case, the lower number in the Snellen fraction is less than 20. For example, if you have 20/15 visual acuity, the smallest letters you can see at 20 feet a person with normal eyesight cannot recognize at 20 feet - they must be closer (15 feet) to recognize them. Your eyesight is 33% better than normal (20/15 = 133%).



Near visual acuity is also described using a Snellen fraction. Although the near test distance is typically 16 inches, letters used on the near test card are appropriately sized to give similar relative measurements of eyesight as the distance Snellen fraction. Therefore, the top number in the Snellen fraction remains 20 for near vision testing, and the fraction is called a "Snellen equivalent."



Sometimes visual acuity is measured using a test distance of 6 meters instead of 20 feet. In these cases, the Snellen fraction "6/6" indicates normal eyesight (6/6 = 20/20), 6/12 is the same relative level of eyesight as 20/40, and so on.