How to use the online eye test
- Calibrate your screen. Hold a bank card or ID card against the screen and resize the rectangle until it matches. This lets the letters be drawn at their true size. You only need to do it once per device.
- Set the viewing distance. The default is 3 metres (10 feet); anything from 2 to 6 metres works. A longer distance lets the test show smaller lines. The mirror trick used by some printed charts is not supported; look at the screen directly.
- Choose the chart type. Letters uses the ten Sloan letters of modern clinical charts; Tumbling E suits children and anyone who does not read Latin letters.
- Run the distance test. Cover your left eye and name each marked letter (or the direction of the E), then repeat for the left eye. A wireless keyboard or a helper enters the answers from across the room.
- Try the near vision test. Hold the device at 40 cm and tap the smallest paragraph you can read comfortably.
- Check your colour vision. The Colour vision tab shows 8 dot plates for a quick red-green and blue-yellow screening.
- Read your results below the test, or print a full chart from the Snellen chart tab.
How to get a fair result
Most errors come from the setup rather than the eyes. Measure the distance with a tape measure, because the letter sizes are calculated for it. Light the room evenly, turn the brightness up and avoid reflections on the screen. Cover one eye with your palm or a piece of card without pressing on it, since pressing can blur that eye for a while afterwards. Do not squint or lean forward. If you wear glasses or contact lenses for distance, keep them on: the useful question is how well you see in daily life.
Each line shows five symbols and you pass it by getting at least three right. The test starts on a medium line, moves to smaller lines while you pass and larger ones when you miss, and reports the smallest line you passed. Guessing is fine.
What 20/20 and 6/6 mean
Visual acuity is written as a fraction. The first number is the testing distance and the second is the distance at which a person with standard vision could just read the same line. 20/20 (in feet) or 6/6 (in metres) means you read at 20 feet what a standard eye reads at 20 feet. 20/40 (6/12) means you need to be at 20 feet to read letters that a standard eye can read from 40 feet, so the letters must be twice as large for you. On the 20/20 line each letter is 5 minutes of arc tall, and its strokes and gaps are 1 minute of arc, about one sixtieth of a degree.
20/20 is a reference point, not perfect vision: many healthy young adults read the 20/16 or even the 20/12.5 line. Clinicians often use logMAR, the logarithm of the smallest detail angle you can see. 20/20 is logMAR 0.0, each line on the chart is a step of 0.1, positive numbers are worse and negative numbers better, so 20/40 is 0.3 and 20/16 is −0.1.
| Feet | Metres | logMAR | Rough meaning |
|---|---|---|---|
| 20/12.5 – 20/16 | 6/3.8 – 6/4.8 | −0.2 to −0.1 | Better than the 20/20 standard |
| 20/20 | 6/6 | 0.0 | Standard normal vision |
| 20/25 – 20/32 | 6/7.5 – 6/9.5 | 0.1 to 0.2 | Mildly reduced |
| 20/40 | 6/12 | 0.3 | A level often used as a driving standard; rules vary by country |
| 20/50 – 20/200 | 6/15 – 6/60 | 0.4 to 1.0 | Clearly reduced; worth an eye exam |
Short-sight, long-sight, presbyopia and astigmatism
Short-sightedness (myopia) means the eye focuses distant objects in front of the retina, usually because the eye is slightly too long. Far things look blurred while close things stay clear.
Long-sightedness (hyperopia) means the eye focuses behind the retina. Younger people can often make up for it by focusing harder, so they may pass a distance test easily and still get tired eyes or headaches when reading. A screen test can easily miss it.
Presbyopia is the gradual loss of the eye's ability to focus up close as the natural lens stiffens with age. It usually becomes noticeable from around the early to mid forties, when small print gets harder to read.
Astigmatism means the cornea or lens is curved unevenly, so some directions are sharper than others and vision can be blurred at every distance. Try the astigmatism test for a quick clock-dial check.
What an online eye test cannot do
A screen test can tell you roughly how sharp your vision is at one distance and whether one eye sees worse than the other. It cannot work out a glasses or contact lens prescription, check eye pressure, look at the retina or optic nerve, or detect conditions such as glaucoma, cataract, diabetic eye disease or macular degeneration, several of which cause no symptoms early on. If the screen cannot draw letters small enough at your distance, the result is shown as "20/20 or better" or similar. For colour vision use the colour blind test, and for distortion in your central vision use the Amsler grid test.
When to see an optometrist
Book an eye exam if either eye scores worse than 20/40 (6/12), if your eyes differ by two lines or more, if your result is worse than last time, or if you have blurred vision, eye strain or headaches when reading. Regular check-ups are worthwhile even when you see well; guidance varies by country, but 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. Seek urgent care for sudden vision loss, a shadow or curtain across your vision, a burst of flashes and new floaters, sudden distortion, or a painful red eye.
Privacy: everything runs in your browser. Your calibration and latest results are kept only in this browser's local storage until you clear them, and nothing is sent to a server. More eye and hearing screenings are on the health tests page.
Frequently asked questions
Is an online eye test accurate?
It can be reasonably close when the screen is calibrated, the distance is measured and the lighting is good, but expect it to differ by about a line from a test on a real chart. Treat it as a screening that tells you whether an eye exam is a good idea, not as a measurement.
What does 20/20 vision mean?
It means that at 20 feet (6 metres) you can read the line that a person with standard vision can read at that distance. The letters on that line are 5 minutes of arc tall. 20/20 is normal, not perfect: many people see better, such as 20/16.
Can I get a prescription from an online test?
No. A prescription needs a refraction by an optometrist or ophthalmologist, who measures how your eyes focus with lenses and checks your eye health at the same time. This test only shows how small a line you can read.
Can this test tell if I am nearsighted or farsighted?
Only as a hint. A weaker distance result with clear near vision fits short-sightedness (myopia); trouble with small print fits presbyopia or long-sightedness (hyperopia). Other causes can look the same. An eye exam gives the real answer.
How far should I sit from the screen?
Use the distance set in the test, 3 metres (10 feet) by default, measured from the screen to your eyes. Anything from 2 to 6 metres works. For the near test, hold the device about 40 cm away.
Does the eye test check colour blindness?
It includes a quick 8-plate colour vision check. For a more reliable screening with 20 plates and a hue arrangement test, take the full colour blind test.
Should I test with glasses on?
Yes, if you normally wear glasses or contact lenses for distance, keep them on so the result reflects your everyday vision. Use reading glasses for the near test if you usually read with them.
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