What Is 20/20 Vision? Visual Acuity Explained

What the numbers mean

The score is a fraction. The top number is the test distance: 20 feet, or 6 meters, roughly the length of an exam room. The bottom number is the distance at which a person with normal vision could read the smallest line you can read.

So 20/20 means you see at 20 feet what a typical person sees at 20 feet. 20/40 means the smallest letters you can read at 20 feet are ones a normal eye can read from 40 feet away; your sight is half as sharp. 20/10 means you can read at 20 feet what most people need to be at 10 feet to read, which is unusually sharp.

The letters on each line are sized so that at the stated distance each part of a letter (the bar of an E, for example) is 1 minute of arc tall, about one sixtieth of a degree. That is the detail a normal eye can just resolve.

Common scores and what they mean

The scores are measured one eye at a time, usually first without glasses and then with them. "Best corrected" acuity, with the right lenses, is what most legal standards use.

Is 20/20 perfect vision?

No. 20/20 is the standard for normal sight, and many healthy people do better. It also only measures sharpness of high-contrast black letters at a distance. You can have 20/20 and still have:

• Long sight or age-related near sight loss (presbyopia), where distance is sharp but close reading is hard. Distance charts do not test reading vision.
• Mild astigmatism that blurs some angles more than others or causes eye strain; see astigmatism explained.
• Color vision deficiency, which an acuity chart does not test at all; see the color vision test.
• Poor contrast sensitivity, trouble in fog, at dusk or with glare, common with early cataracts.
• Side vision loss, as in glaucoma, which can be advanced before central sharpness drops.
• Eye disease with no change in sight yet. Many conditions are found only in a full eye exam.

How visual acuity changes with age

Babies are born with blurry sight, around 20/400, and reach adult levels of sharpness by about age five. That is why young children are screened with picture or tumbling E charts rather than letters, and why a lazy eye (amblyopia) needs treatment early, while the visual system is still developing.

Healthy young adults often see 20/15 or better. Distance acuity usually stays good through middle age, while close focus fades from the early forties as the lens stiffens. After about 60, cataracts, a smaller pupil and changes in the retina tend to lower acuity and contrast, so many older adults sit at 20/25 to 20/30 even with glasses.

A sudden drop at any age is different from slow change. Blurred, distorted or lost sight that comes on over hours or days needs an urgent eye exam.

20/20 and driving

Driving standards vary by country and state, but many sit around 20/40 (6/12) with both eyes open, with glasses or contact lenses allowed. The UK, for example, asks drivers to read a number plate at 20 meters and to meet at least 6/12 on the Snellen scale. Many places also have a minimum field of vision.

If you only reach the standard with glasses, your licence usually says so and you must wear them to drive. Check the rules where you live; this guide is not legal advice.

How to check your visual acuity

The eye test on this site shows a calibrated tumbling E chart. You set the screen size and your distance, then say which way each E points, one eye at a time. It gives an estimate in feet, meters and decimal.

For a fair result: calibrate the screen size with a card or ruler, sit at the stated distance, use good room light without glare on the screen, cover one eye with your palm without pressing, and wear your glasses if you want to check your corrected sight.

An online test is a screening check, not an exam. Screens vary in contrast and sharpness, and you cannot measure your own eye pressure, focus or retina. If your result is worse than 20/30 in either eye, if one eye is much worse than the other, or if your sight has changed, book an eye exam.