What does 6/6 (20/20) vision mean? 6/9 and 6/12 explained, and why they are not a glasses prescription

6/6 vision is normal sharpness of sight: standing 6 metres from an eye chart, you can read the line that a person with typical healthy sight reads from 6 metres. It is the same as 20/20 in countries that measure in feet, since 20 feet is roughly 6 metres. The first number is your distance from the chart; the second is the distance from which a typical eye reads the smallest line you managed. So 6/12 means that from 6 metres you read only what a typical eye reads from 12, while 6/9 is only slightly below typical. The fraction measures the sharpness of distance vision only: it does not judge the health of the whole eye, and it is not your glasses prescription.

How the eye chart works

The familiar chart of letters that shrink line by line is called a Snellen chart, after the Dutch eye doctor who designed it in 1862. Each letter is drawn so that, at the distance marked beside its line, it fills a set angle of your field of view: 5 minutes of arc, or one twelfth of a degree, with each stroke of the letter one fifth of that, or 1 minute of arc. The test is done at 6 metres because light from that distance reaches the eye almost parallel, so a typical eye needs virtually no focusing effort and the result reflects its sharpness at rest. Smaller clinics often show the chart in a mirror, so a room 3 metres long is enough while the optical distance stays at 6 metres. Each eye is tested on its own with the other covered, sometimes followed by both eyes together, first without glasses and then with them.

• Uncorrected: what the eye sees without glasses or contact lenses, sometimes written as sc.
• Corrected: with your current glasses, sometimes written as cc. The best result with the ideal lenses in the test frame (best-corrected acuity) is the number that matters most for judging eye health.
• The pinhole test: if you read more lines looking through a tiny hole, the blur is most likely a focusing (refractive) error that the right lens can correct; if not, the doctor looks for another cause.
• Children who cannot read letters yet are tested with pictures or shapes, or with a letter E turned in different directions, pointing to the way it faces.

What 6/9, 6/12 and 6/5 mean

The rule is simple: the larger the second number, the weaker the sharpness. 6/9 is slightly below typical, 6/12 means you have to come twice as close as a typical eye to read the same line, and 6/60 means that from 6 metres you read only the big letter a typical eye reads from 60. The second number can also be smaller than 6: 6/5 or 6/4 means sharper than typical, and many healthy young adults can read smaller lines than the 6/6 line. That is why 6/6 is more accurately described as normal sight rather than perfect sight. You may also find the same result written in different formats, depending on the clinic or the country:

• In feet: 20/20 equals 6/6, 20/40 equals 6/12, and 20/200 equals 6/60.
• Decimal: common on European reports, where 1.0 equals 6/6, 0.5 equals 6/12 and 0.1 equals 6/60.
• LogMAR: used in research and in many clinics, where 0.0 equals 6/6, 0.3 equals 6/12 and 1.0 equals 6/60, and the lower number is the better one.
• A minus after the fraction, such as 6/9 -2, means you read that line but missed 2 of its letters.

6/6 is not your glasses prescription

This is where the most common confusion lies. Visual acuity is a result: how much detail you see. The prescription is the remedy: the lens power, in dioptres, that brings your sight to its sharpest. Two people with the same prescription of -1.50 can have different sharpness without glasses, and no fixed table converts one into the other, because acuity also depends on pupil size, lighting, how clear the cornea and the eye's natural lens are, and the health of the retina. Farsightedness is the clearest example: a young adult with +1.50 may read 6/6 at a distance without glasses, because the eye quietly makes up the difference by focusing, and still get headaches when reading. So 6/6 without glasses does not always mean you need no glasses, and a prescription does not tell you how much you see without them. A few common ways to read your result:

• 6/6 with glasses: your correction is doing its job.
• Below 6/6 even with the best lenses: the question is no longer about glasses, and the doctor looks for another cause, in the eye itself or in how the brain processes vision, such as a lazy eye, a cataract or a problem in the retina.
• A drop in sharpness that new lenses do not bring back: it needs an explanation from a doctor, not just a new pair.
• A sudden drop in vision: do not wait for a routine appointment; get seen the same day, and if you suddenly cannot see from one or both eyes, go straight to the emergency department.

What 6/6 does not tell you

The chart measures one thing: the sharpness of central vision for crisp black letters on a white background. The American Academy of Ophthalmology points out that 20/20 is not perfect vision, since some people see more sharply than that. And the chart says nothing about several other abilities you rely on every day:

• Side (peripheral) vision: glaucoma can eat into it for years while you still read the bottom lines of the chart.
• Contrast: spotting a grey car in dust or a step in dim light can be poor even with a 6/6 result.
• Near focus: at 45 you may read 6/6 at a distance and still hold your phone at arm's length to read a message.
• Eye coordination and depth perception: how the two eyes work together.
• Colour vision: measured with separate tests.
• Eye health: the pressure inside the eye, the eye's natural lens and the retina are checked by examination, not by reading letters.

Near vision has its own measurement too: it is taken at reading distance with a special card and is often written as N5 or N8, where the smaller number means smaller print. That is why a full eye exam is much broader than the letter chart, and why a school or driving-licence vision screening, useful as it is, does not replace one.