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Visual Acuity: What It Is, How It Is Measured and What 20/20 Means
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Visual acuity (noun): visual acuity is the sharpness of your vision, meaning how small a detail you can pick out at a set distance. It is measured with an eye chart and written as a fraction such as 20/20 (feet) or 6/6 (metres), or as a decimal or logMAR number. Clinicians record it for each eye separately, with and without glasses or contact lenses.
In short: visual acuity of 20/20, also written 6/6 or 1.0, is the standard for normal distance sharpness: you see at 20 feet (6 metres) what an average person sees at that distance. It is not "perfect vision". A reduced result can come from a focusing error, cataract, a corneal or retinal problem, or lazy eye, and an eye surgeon such as Prof. Dr. Ahmet Satıcı, Doç. Dr. Muhammet Derda Özer or Op. Dr. Aysan Bayram can find the cause and treat it.
Visual acuity: key facts
- What it measures
- Clarity of detail vision, usually at distance; near acuity is tested separately
- Normal result
- 20/20 in feet, 6/6 in metres, 1.0 as a decimal, 0.00 in logMAR
- How to read the fraction
- The bottom number is the distance at which an average person reads the same line, so 20/40 is poorer than 20/20
- Better than 20/20
- Possible: 20/15 means you read at 20 feet what an average person needs to be 15 feet from
- Typical chart
- Snellen chart (letters), ETDRS/logMAR chart (clinics and research), tumbling E or picture charts (children and non-readers)
- What it does not measure
- Side vision, colour, depth perception, contrast, or eye disease such as glaucoma
- Who treats low acuity
- An ophthalmologist; at Laser Istanbul Clinic, Prof. Dr. Ahmet Satıcı, Doç. Dr. Muhammet Derda Özer or Op. Dr. Aysan Bayram
What does 20/20 vision mean?
20/20 vision means you can see what an average person can see on an eye chart when standing 20 feet away, according to the American Academy of Ophthalmology (AAO). The top number is your testing distance. The bottom number is the distance at which a person with typical sight could read the same line. So 20/40 means you need to be at 20 feet to read what most people can read from 40 feet, and 20/15 means your sharpness is better than average.
Outside the United States the same idea is written in metres: 6/6 is the equivalent of 20/20. In the UK, Ireland, Australia and much of Europe you will see 6/6, 6/9 or 6/12 on a prescription or eye test report, while the US uses 20/20, 20/30 or 20/40.
Is 20/20 perfect vision? No. The AAO states that 20/20 is normal clarity and sharpness, not perfect vision. Vision sharper than 20/20 is possible, and healthy young eyes often read better than 6/6. The AAO also reports that only about 35% of adults see 20/20 without any correction, while about 75% reach it with glasses, contact lenses or surgery. A 20/20 result also says nothing about side vision, colour vision, depth perception or eye health.
Visual acuity chart: Snellen, decimal and logMAR notations
The same acuity can be written in several ways. The table below converts common values. The 20/200, 20/100, 20/40, 6/6, 6/12 and 6/60 rows follow the standard conversion table; the other rows are calculated with the same formula (decimal = the first number divided by the second; logMAR = the logarithm of the minimum angle of resolution, which is 1 divided by the decimal).
| Snellen (feet) | Snellen (metres) | Decimal | logMAR | In plain words |
|---|---|---|---|---|
| 20/10 | 6/3 | 2.00 | −0.30 | Exceptionally sharp |
| 20/15 | 6/4.5 | 1.33 | −0.12 | Better than average |
| 20/20 | 6/6 | 1.00 | 0.00 | Normal acuity |
| 20/25 | 6/7.5 | 0.80 | 0.10 | Just below normal |
| 20/30 | 6/9 | 0.67 | 0.18 | Mildly reduced |
| 20/40 | 6/12 | 0.50 | 0.30 | Reduced: you need to be at 20 feet for what others read at 40 |
| 20/50 | 6/15 | 0.40 | 0.40 | Noticeably reduced |
| 20/60 | 6/18 | 0.33 | 0.48 | Clearly reduced |
| 20/100 | 6/30 | 0.20 | 0.70 | Markedly reduced |
| 20/200 | 6/60 | 0.10 | 1.00 | Severely reduced |
For an instant conversion between 20/20, 6/6, decimal and percentage-style values, use our free tool.
Why "percent vision" is misleading. Acuity is a ratio of distances, not a percentage of sight. Someone with 20/40 does not have "50% vision" in the everyday sense, and a decimal such as 0.5 is only a convenient way of writing the same ratio.
What do 6/9, 6/12 and 6/60 mean?
- 6/9 (20/30, decimal 0.67): at 6 metres you read what an average person reads at 9 metres. It is a mild reduction and can have many causes, including a small uncorrected prescription.
- 6/12 (20/40, decimal 0.50): you read at 6 metres what an average person reads at 12 metres.
- 6/60 (20/200, decimal 0.10): you read at 6 metres only what an average person reads at 60 metres, which is usually the largest letter on the chart.
How is visual acuity tested?
A visual acuity test is quick, painless and part of every eye examination. The National Eye Institute (NEI) describes it as reading letters at a distance and up close to see how clearly you see.
The standard Snellen chart. According to the AAO, the chart has 11 rows of capital letters, starting with one large letter and getting smaller. You stand 20 feet away (6 metres), cover one eye and read the smallest line you can. Dutch ophthalmologist Hermann Snellen developed the chart in the 1860s; each letter is built on a 5 × 5 grid, so the whole letter spans 5 arcminutes and each stroke 1 arcminute.
The logMAR / ETDRS chart. Used in clinics and clinical research, this chart has five letters on every line, with the same spacing between lines. Each line is worth 0.1 logMAR units and each letter 0.02, so the score can be counted letter by letter instead of line by line. It was designed by Ian Bailey and Jan Lovie-Kitchin in 1976 as a more standardised alternative to the Snellen chart. A 2026 comparison of a digital chart with the ETDRS chart found a mean difference of only −0.03 logMAR, which the authors report as clinically equivalent.
Charts for children and non-readers. The tumbling E chart (you say or show which way the "E" points) and picture charts with houses, flowers or objects are used for young children and people who cannot name letters. Children aged 3 to 4 should see the 20/40 line, and from age 5 the 20/30 line, according to the AAO.
Which kind of acuity is recorded? Three versions are common, and the difference matters for treatment decisions:
- Uncorrected: without glasses or contact lenses. It shows how much you rely on correction.
- Presenting: with whatever you normally wear.
- Best-corrected: with the best possible lens correction found during the test. If best-corrected acuity is still low, the cause is not only a prescription.
When letters are too big to read, clinicians record whether you can count fingers, see hand movement or perceive light.
A home chart is only a screen. The AAO says home tests are no substitute for an exam by an eye care provider. Use them as a prompt to book an appointment, not as a diagnosis.
What affects visual acuity?
Visual acuity depends on the eye's optics and on the nerve pathway to the brain. It can fall when any part of that chain is affected.
| Cause | What happens | Typical clue |
|---|---|---|
| Refractive error (short-sightedness, long-sightedness, astigmatism) | Light is not focused sharply on the retina | Vision improves with glasses or contact lenses. The WHO calls uncorrected refractive error a leading cause of vision impairment in all countries |
| Cataract | The eye's natural lens becomes cloudy | Blurred or dim vision, glare from oncoming headlights at night, faded colours (AAO) |
| Cornea problems, such as keratoconus | The cornea thins and bulges into a cone, distorting the image | Blur and distortion, and frequent prescription changes (AAO) |
| Retina and macula disease, such as age-related macular degeneration and diabetic retinopathy | The light-sensing tissue at the back of the eye is damaged | Blur that glasses do not fix |
| Amblyopia ("lazy eye") | In childhood the brain learns to ignore one eye, so it does not develop normal sharpness | One eye reads fewer lines than the other even with glasses |
| Glaucoma and optic nerve disease | Damage to the nerve that carries the image to the brain | Eye charts do not detect glaucoma, so a normal chart result can hide the disease |
The WHO estimates that at least 2.2 billion people worldwide have a near or distance vision impairment, and for at least 1 billion of them it could have been prevented or has yet to be addressed. Its fact sheet lists refractive error, cataract, diabetic retinopathy, glaucoma and age-related macular degeneration among the leading causes.
Why a normal acuity result does not rule out eye disease
Eye charts do not detect conditions such as glaucoma or retinal problems, and they do not measure loss of peripheral vision, as the AAO notes. A full eye examination adds a visual field test, pupil and eye-muscle checks, eye-pressure measurement (tonometry) and a dilated examination of the back of the eye (NEI).
Visual acuity categories: what counts as reduced vision?
For public-health reporting, the WHO classifies distance vision impairment by presenting acuity, which is the acuity with the glasses or lenses a person normally uses. A 2026 paper in Ophthalmic Epidemiology applying these categories defines mild impairment as presenting acuity from 6/18 up to (but not including) 6/12, moderate-to-severe impairment as worse than 6/18 down to 3/60, and blindness as worse than 3/60 (or a visual field narrower than 10° around fixation). Near vision impairment is defined as near acuity worse than N6 (or M 0.8) at 40 cm.
| Category | Presenting distance acuity |
|---|---|
| Normal or near-normal | 6/12 or better |
| Mild impairment | Worse than 6/12, up to 6/18 |
| Moderate to severe impairment | Worse than 6/18, down to 3/60 |
| Blindness | Worse than 3/60 |
These bands describe how common vision problems are in populations. Your own eye doctor looks at the whole picture, including best-corrected acuity, visual field and the cause.
Symptoms: how do you know your visual acuity has dropped?
Gradual loss is easy to miss because the brain adapts. The NHS lists these signs of vision loss:
- changes to your vision
- difficulty reading, watching TV or seeing people's faces clearly
- eyes that are uncomfortable in, or sensitive to, bright light
- difficulty seeing at night and in low light
Other warning symptoms include blurred vision, double vision, flashing lights, dark shadows crossing your vision and eye pain.
In children, a screening chart is a useful check: the AAO expects children aged 3 to 4 to see the 20/40 line and children from age 5 to see the 20/30 line.
Can visual acuity be improved?
It depends on the cause. Treatment targets the reason for the blur, not the chart number itself.
| Cause | Typical approach |
|---|---|
| Refractive error | Glasses or contact lenses; or laser vision correction (LASIK, SMILE, PRK) or an implantable lens (ICL) for suitable adults with a stable prescription |
| Cataract | Surgery to remove the cloudy lens and replace it with an artificial intraocular lens; surgery is the only way to remove a cataract (AAO) |
| Presbyopia (age-related near-focusing difficulty) | Reading glasses or multifocal lenses; lens replacement with a premium or trifocal lens for suitable patients |
| Keratoconus | Corneal cross-linking, which strengthens the cornea and aims to prevent further bulging; special contact lenses; transplant in severe cases (AAO) |
| Retinal disease | Specialist retinal treatment, depending on the condition |
| Amblyopia in children | Making the child use the weaker eye, by patching, blurring drops or special lenses; best started before age 7 or 8 (AAO) |
Surgery does not guarantee a particular number. As an example, the AAO says about 9 in 10 people who have LASIK end up between 20/20 and 20/40 without glasses or contact lenses; it also lists side effects, including dry eye, glare and halos, and says some people need further treatment. Candidates need a stable prescription and healthy, thick-enough corneas.
Contact lenses and acuity. Soft, rigid gas-permeable, hybrid and scleral lenses correct acuity in different ways. The AAO notes that rigid gas-permeable lenses can give sharper vision than soft lenses for moderate-to-severe astigmatism and keratoconus, and that scleral lenses are designed for irregular corneas from scarring or keratoconus.
Orthokeratology. Ortho-k uses rigid lenses, mostly worn during sleep, to temporarily reshape the cornea so that vision is clearer during the day. The effect fades if you stop wearing the lenses, and the AAO notes an increased risk of infection.
Treatment with us: if your acuity is reduced and you want to know why, the assessment is led by the surgeon whose specialty fits your eyes: Prof. Dr. Ahmet Satıcı (cataract, premium and trifocal lenses, laser, ICL), Doç. Dr. Muhammet Derda Özer (cornea, keratoconus, retina; "Doç." means Associate Professor) or Op. Dr. Aysan Bayram (cataract, laser, retinal disease). Surgery takes place in a fully equipped, Ministry of Health–licensed operating theatre in Istanbul.
When to see a doctor about your visual acuity
Seek same-day care from an eye doctor or eye emergency service if you have:
- a sudden drop in vision in one or both eyes
- flashing lights, or a dark shadow crossing your vision
- eye pain with redness and blurred vision
- sudden double vision
Book a routine appointment if:
- you cannot read the 20/20 line (6/6) on a home or screening chart, or the two eyes give clearly different results
- distance or reading vision is slowly getting worse, or your prescription keeps changing
- you are struggling at night, with glare, or with faces and screens
- your child cannot read the line expected for their age, or one eye seems to turn
The AAO recommends a baseline eye examination at age 40. The NEI suggests exams every 1–2 years for people over 60 or with a family history of glaucoma, and annually for people with diabetes or high blood pressure.
If you already have a diagnosis, such as cataract or keratoconus, and want a second opinion, you can send your scans and prescription for review before you travel.
Frequently asked questions
Visual acuity is the sharpness of your vision: how small a detail you can see at a given distance. It is measured with an eye chart and written as a fraction such as 20/20 or 6/6, as a decimal, or in logMAR.
Normal visual acuity is 20/20 (6/6, decimal 1.0, logMAR 0.00). The AAO describes it as normal clarity and sharpness, not perfect vision, and better than 20/20 is possible.
6/6 is the metric version of 20/20. At 6 metres you can read what an average person reads at 6 metres.
6/9 (20/30) means you read at 6 metres what an average person reads at 9 metres; 6/12 (20/40) means what an average person reads at 12 metres. Both are below normal. An eye test shows whether a new prescription or another treatment would help.
No. It describes sharpness at a distance only. Side vision, colour vision, depth perception, contrast and eye health are tested separately, and a full examination is needed to detect diseases such as glaucoma.
Snellen charts have varying numbers of letters per line. logMAR (ETDRS) charts have five letters on every line, with equal spacing, so each line is worth 0.1 and each letter 0.02 logMAR. This gives a more consistent score, which is why clinics and research use it.
20/20 equals 6/6 and 1.0. To convert, divide the first number by the second: 20/40 is 0.5, which is 6/12. Our converter does this for you.
If the cause is a prescription, glasses or contact lenses restore acuity. In children with lazy eye, patching or blurring drops can help if started early. Cataract and some corneal and retinal problems need medical or surgical treatment, so the first step is finding the cause.
Many people reach 20/20 to 20/40 without glasses after LASIK (about 9 in 10, according to the AAO), but no result can be guaranteed. A pre-operative assessment decides whether your eyes are suitable.
See an eye doctor if you notice any change. Beyond that, the AAO recommends a baseline examination at 40, and the NEI advises more frequent checks for people over 60, with diabetes or high blood pressure, or with a family history of glaucoma.
Sources for the facts on this page
- American Academy of Ophthalmology. What Does 20/20 Vision Mean? https://www.aao.org/eye-health/tips-prevention/what-does-20-20-vision-mean
- American Academy of Ophthalmology. All About the Eye Chart. https://www.aao.org/eye-health/tips-prevention/eye-chart-facts-history
- American Academy of Ophthalmology. Home Vision Tests for Children and Adults. https://www.aao.org/eye-health/tips-prevention/home-eye-test-children-adults
- National Eye Institute. Get a dilated eye exam. https://www.nei.nih.gov/learn-about-eye-health/healthy-vision/get-dilated-eye-exam
- Wikipedia. Visual acuity (conversion table). https://en.wikipedia.org/wiki/Visual_acuity
- Wikipedia. LogMAR chart (0.1 per line, 0.02 per letter, Bailey and Lovie 1976). https://en.wikipedia.org/wiki/LogMAR_chart
- Wikipedia. Snellen chart (5-arcminute letters, 1-arcminute strokes). https://en.wikipedia.org/wiki/Snellen_chart
- Yu Z. Digital chart versus ETDRS chart comparison. Upsala Journal of Medical Sciences 2026. PMID 41783464. https://pubmed.ncbi.nlm.nih.gov/41783464/
- World Health Organization. Blindness and vision impairment (fact sheet). https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment
- Prevalence of Vision Impairment in South-East Asia and Oceania in 2020: Magnitude and Temporal Trends. Ophthalmic Epidemiology 2026. PMID 41253498 (states the category thresholds). https://pubmed.ncbi.nlm.nih.gov/41253498/
- Wikipedia. Visual impairment (near vision N6 / M 0.8 at 40 cm, WHO). https://en.wikipedia.org/wiki/Visual_impairment
- NHS. Vision loss. https://www.nhs.uk/conditions/visual-impairment/
- American Academy of Ophthalmology. What Is Amblyopia (Lazy Eye)? https://www.aao.org/eye-health/diseases/amblyopia-lazy-eye
- American Academy of Ophthalmology. What Are Cataracts? https://www.aao.org/eye-health/diseases/what-are-cataracts
- American Academy of Ophthalmology. What Is Keratoconus? https://www.aao.org/eye-health/diseases/what-is-keratoconus
- American Academy of Ophthalmology. LASIK Laser Eye Surgery. https://www.aao.org/eye-health/treatments/lasik
- American Academy of Ophthalmology. Contact Lens Types. https://www.aao.org/eye-health/glasses-contacts/contact-lens-types
- American Academy of Ophthalmology. What Is Orthokeratology? https://www.aao.org/eye-health/glasses-contacts/what-is-orthokeratology
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