You can check your eyesight at home by reading a correctly sized letter chart from a measured distance, one eye at a time, in good light and with your usual glasses on. The result is your visual acuity, written as a fraction such as 20/20 in feet or 6/6 in metres: 20/20 means you can read at 20 feet (6 metres) what a person with standard vision reads at that distance. A home check is a useful screening, but it cannot measure a prescription or examine the health of your eyes, so it never replaces an eye examination.
- 20/20 (6/6) is the standard reference for normal sharpness, not perfect vision; many healthy adults see better than 20/20.
- The second number grows as vision gets worse: 20/40 (6/12) means you need letters twice as large as the 20/20 line.
- A fair home test needs a calibrated chart, a measured distance, even lighting and one eye covered at a time.
- Home tests can miss long-sightedness and cannot detect glaucoma, cataract or diabetic eye disease.
- Sudden vision loss, a shadow or curtain over your vision, a burst of flashes and floaters, or a painful red eye need urgent care, not an online test.
What is visual acuity?
Visual acuity measures how much fine detail your eye can resolve at a set distance: the smallest line of letters you can read reliably. It describes sharpness only, not colour vision, side vision or eye health. Distance acuity is usually tested at 6 metres (20 feet) or a shorter distance with scaled letters; near vision is checked separately at about 40 cm with print sizes labelled N5, N6, N8 and so on.
What does 20/20 vision mean?
Visual acuity is written as a fraction. The first number is the testing distance; the second is the distance at which a person with standard vision could just read the smallest line you managed. The United States uses feet (20/20); the United Kingdom and many other countries use metres (6/6). Some use a decimal form, where 6/6 is 1.0 and 6/12 is 0.5.
- 20/20 (6/6): at 20 feet you read what a standard eye reads at 20 feet. This is the conventional benchmark for normal acuity.
- 20/40 (6/12): at 20 feet you can only read letters that a standard eye could read from 40 feet away, so the letters have to be twice as large for you.
- 20/200 (6/60): the letters must be ten times as large. This is the big letter at the top of many charts.
The benchmark is based on the size of detail the eye can separate. On the 20/20 line each letter is five minutes of arc tall, and the strokes and gaps within it are one minute of arc, which is one sixtieth of a degree. 20/20 is a reference point, not a maximum: many healthy young adults read one or two lines smaller.
Many driving standards sit around 20/40 (6/12), with correction if needed, and in the United States "legal blindness" is commonly defined as 20/200 or worse in the better eye with the best correction, or a severely restricted field. Rules vary by country, so check the official ones rather than relying on a home test.
Snellen charts vs logMAR charts
The familiar chart with one large letter at the top is the Snellen chart, introduced by the Dutch ophthalmologist Herman Snellen in 1862. It is quick, but the top lines have one or two letters while lower lines have many, and the size steps between lines are uneven, so small changes are harder to measure consistently.
Clinical research and many clinics use logMAR charts such as the ETDRS chart instead. Every line has five letters, spacing is proportional to letter size, and each line is a fixed step of 0.1 log units. The score is the logarithm of the minimum angle of resolution (MAR): 20/20 is logMAR 0.0, 20/40 is 0.3 and 20/200 is 1.0. Lower is better, and negative values such as −0.1 (20/16) beat the benchmark.
| Feet | Metres | Decimal | logMAR | Rough meaning |
|---|---|---|---|---|
| 20/16 | 6/4.8 | 1.25 | −0.1 | Better than the 20/20 benchmark |
| 20/20 | 6/6 | 1.0 | 0.0 | Standard normal acuity |
| 20/25 | 6/7.5 | 0.8 | 0.1 | Slightly reduced |
| 20/32 | 6/9.5 | 0.63 | 0.2 | Mildly reduced |
| 20/40 | 6/12 | 0.5 | 0.3 | Around many driving standards; rules vary |
| 20/63 | 6/19 | 0.32 | 0.5 | Clearly reduced; book an eye exam |
| 20/200 | 6/60 | 0.1 | 1.0 | Severely reduced |
For children and people who do not read the Latin alphabet, charts use symbols such as the Tumbling E, where you say which way the E points.
How to check your eyesight at home properly
Most errors in a home test come from the set-up rather than the eyes, because the letters must be the correct physical size for your distance. Our online eye test sizes them for you once you calibrate the screen with a bank card, and offers letter and Tumbling E charts plus a near vision check. Whichever chart you use:
- Calibrate or print at the right size. On paper, print at 100% ("Actual size") and check a reference line with a ruler.
- Measure the distance with a tape measure; three metres (10 feet) suits most homes.
- Light the room evenly and avoid glare, reflections and dim light.
- Wear your usual distance glasses or contact lenses, because the useful question is how well you see in daily life.
- Test one eye at a time. Cover the other with your palm or a card without pressing, which can blur it afterwards.
- Do not squint or lean forward; squinting makes vision look better than it is.
- Count a line as read if you get most of it right, commonly at least three of five letters. Guessing is allowed.
- Note the date and result for each eye. A change over time, or a two-line difference between eyes, says more than a single number.
Short-sight, long-sight, presbyopia and astigmatism
Most reduced acuity in healthy eyes comes from a refractive error: the eye does not focus light sharply on the retina.
| Condition | What happens | Typical effect | Does a distance chart pick it up? |
|---|---|---|---|
| Short-sightedness (myopia) | Light focuses in front of the retina, usually because the eye is slightly too long | Distant things look blurred; close things stay clear | Usually, yes |
| Long-sightedness (hyperopia) | Light focuses behind the retina | Younger people can focus through it, but may get tired eyes or headaches when reading | Often not; it can be missed |
| Presbyopia | The natural lens gradually stiffens with age and loses focusing range | Small print gets harder to read, usually becoming noticeable in the early to mid forties | No; a near vision test shows it |
| Astigmatism | The cornea or lens is curved unevenly, so different directions focus at different distances | Blurred or stretched vision at all distances, eye strain | Partly; a clock dial test can hint at the direction |
These often occur together, and almost everyone develops presbyopia eventually. Our astigmatism test uses the classic clock dial: if some lines consistently look darker, astigmatism is one possible reason. It cannot measure the amount; only a refraction by an optometrist can.
Checking your central vision with an Amsler grid
An acuity chart shows how sharp your vision is, but not whether straight lines look straight. The Amsler grid fills that gap: a grid of 5 mm squares, usually 10 cm wide, with a centre dot. Held at about 30 cm it covers roughly the central 20 degrees of vision, served by the macula, which handles reading and detail.
Cover one eye, look only at the dot and notice whether any lines look wavy, blurred or missing. Distortion or gaps can be a sign that the macula has changed, for example in age-related macular degeneration (AMD), and eye doctors often ask people with macular conditions to check each eye regularly. Our Amsler grid test lets you mark affected areas and compare results over time. Have any new distortion checked promptly, and a sudden change the same day.
What about colour vision?
You can have 20/20 vision and still confuse reds and greens. Our colour blind test uses dot plates and a hue arrangement task to screen for red-green and blue-yellow deficiencies, and the guide to types of colour blindness explains what protan, deutan and tritan results mean.
What a home eye test cannot do
A well set-up home test shows roughly how sharp your vision is at one distance, whether one eye sees worse, and whether your vision has changed. That is a small part of an eye examination. A home or online test cannot:
- work out a glasses or contact lens prescription;
- reliably detect long-sightedness, especially in children and young adults who can focus through it;
- measure eye pressure or examine the optic nerve, which is how glaucoma is found, often before you notice any symptoms;
- examine the retina for diabetic eye disease or early macular degeneration;
- check eye alignment or a "lazy eye" (amblyopia) in children, or your full field of vision.
Screen results can also differ by about a line from a clinical chart. Treat them as a prompt, not a measurement.
When should you book an eye exam?
Book an appointment with an optometrist (optician) or eye doctor if any of the following applies:
- either eye reads worse than 20/40 (6/12) with your usual glasses;
- your two eyes differ by two lines or more;
- your result is worse than the last time you checked;
- you have blurred vision, eye strain or headaches when reading or using screens;
- straight lines look wavy on an Amsler grid, or a patch of your vision looks blurred or missing.
Regular eye exams are worthwhile even when you see well, because several eye diseases cause no symptoms early on. A common recommendation is every two years for adults, and more often for children, older people and anyone with diabetes or a family history of glaucoma; local guidance varies. Children's vision should always be checked professionally.
More screenings, including hearing, are on the health tests page.
Frequently asked questions
Is 20/20 vision perfect vision?
No. 20/20 (6/6) is the benchmark for normal distance sharpness, and many healthy people see better. It says nothing about colour vision, side vision, near focus or eye health.
Is 20/40 vision bad?
20/40 (6/12) means you need letters twice as large as the 20/20 line. It is around many driving standards, but if you score it with your current glasses, an eye exam is worthwhile because better correction may be possible.
Can I test my eyes for glasses online?
You can check whether your vision seems reduced, but no online test can produce a reliable prescription or check eye health. If your result is below 20/40, or worse than before, book an eye exam.
How far away should I stand from an eye chart?
Stand at the distance the chart was designed for, usually 6 metres (20 feet) or 3 to 4 metres with smaller letters. For an on-screen chart, calibrate the screen and enter your measured distance.
How often should I have an eye test?
A common recommendation is every two years for adults, but follow your optometrist's and local health service's advice. Children, older people, people with diabetes and those with a family history of glaucoma often need checks more often.