Read this first
This is a screening test on an ordinary screen. It gives a rough idea of how sharply each eye sees at a distance. It cannot find eye diseases, does not measure your glasses prescription and does not replace an exam by an optometrist or ophthalmologist. It does not check color vision either; the color blind test does that. Do not use it to check a child's eyes: children need a professional test, because problems such as a lazy eye are easy to miss and must be treated early.
How the eye test works
Step 1: calibrate the screen. Screens differ in how many pixels make a millimetre, so the test first needs to know your screen's scale. Hold a card in the standard bank-card size (85.6 × 53.98 mm, the ID-1 format used by bank cards and most ID cards) against the screen and move the slider until the outline matches the card exactly. Then press The card matches. The result is saved in this browser, so you only do it once per device.
Step 2: pick a distance. Choose 50 cm, 1 m, 2 m or 3 m and measure it. The letter size is calculated for that distance, so being closer or farther than you said changes the result. A distance is greyed out when the screen cannot draw small enough letters for it. At 2 m and 3 m, use the arrow keys on a keyboard or ask someone to tap the answers you call out. At 50 cm, people over about 40 may score lower because near focus weakens with age. If small print on a screen is what bothers you, the reading speed test shows how fast you read.
Step 3: test each eye. Cover your left eye with your palm, without pressing, and look at the letter E. It is turned up, down, left or right. Choose the direction its bars point with the arrow buttons or the arrow keys, or press I can't tell. Then repeat with the right eye covered. Wear your usual distance glasses or contact lenses if you have them.
How letter size and lines are calculated
On a standard chart, the 20/20 letter (6/6 in metres, 1.0 in decimal) is 5 minutes of arc tall and each bar is 1 minute of arc thick. Herman Snellen set this standard in his 1862 chart. The test draws the E at exactly that angle for your distance: 1.45 mm tall at 1 m and 4.36 mm at 3 m.
Each smaller or larger line differs by 0.1 logMAR, the logarithm of the minimum angle of resolution. This is the step size of the logMAR charts introduced by Ian Bailey and Jan Lovie (1976) and used in the ETDRS chart (Ferris and colleagues, 1982). The test runs from logMAR 1.0 (20/200, decimal 0.1) to −0.2 (20/12.5, decimal 1.6), using only the lines your screen can draw at the distance you picked.
The order of lines:
- You start on the 20/80 line (decimal 0.25).
- Each line shows up to 5 letters. 3 correct answers pass the line, 3 wrong answers fail it.
- If you pass, the next line is smaller. If you fail before passing any line, the test goes to larger lines until you pass one.
- Your result for each eye is the smallest line you passed.
Reading your result
The result card shows each eye as an approximate Snellen fraction and a decimal, for example "≈ 20/25 · 0.8", plus the distance you used. The score saved in your history is the decimal acuity of the better eye, and the comparison band marks 1.0, Snellen's 20/20 standard.
- 1.0 or better: the better eye read the 20/20 line or smaller. Elliott, Yang and Whitaker (1995) showed that many healthy adults see better than 6/6, so 20/20 is a reference, not a ceiling.
- 0.5 to 0.8: the better eye read lines between 20/40 and 20/25. Glasses that need updating, the wrong distance or a dim screen can explain this. Blur that is stronger in one direction can also come from astigmatism, which the astigmatism test checks.
- Below 0.5: worse than 20/40. Check the distance and calibration and book an eye exam. In the WHO's ICD-11 classification, distance vision worse than 6/12 (20/40) in the better eye counts as at least mild visual impairment, and in the European Union an ordinary car licence requires at least 0.5 with both eyes (Directive 2006/126/EC, as amended by 2009/113/EC).
- Smallest line on the screen: if you passed the smallest line your screen can draw, your acuity may be better still. Choose a larger distance to measure further.
If your two eyes differ by two lines or more, the card says so. That is worth mentioning at an eye exam.
Why an online result is approximate
- The calibration depends on how carefully you matched the card, and the distance on how carefully you measured it.
- Screens show small letters with fewer pixels than a printed chart, and brightness and contrast vary. Sharpness at full contrast is also only part of vision; the contrast sensitivity test checks how faint a letter you can read.
- The test uses one optotype (the Tumbling E) and a short staircase, so chance plays a bigger role than with a full chart.
- Squinting, leaning in or peeking with the covered eye all make the result look better than it is.
When to see an eye doctor
Have a full eye exam if your result is below 0.5 in either eye, if your eyes differ by two lines or more, if you have not had an exam for a year or two, or if you notice blur, headaches or eye strain. Go the same day if vision drops or blurs suddenly, if a curtain or shadow covers part of your view, if you see new flashes of light with new floaters, or if your eye is painful and red.
More checks like this are collected on our vision tests page.
Sources
- Snellen, H. (1862). Probebuchstaben zur Bestimmung der Sehschärfe. Utrecht: Van de Weijer.
- Bailey, I. L., & Lovie, J. E. (1976). New design principles for visual acuity letter charts. American Journal of Optometry and Physiological Optics, 53(11), 740–745.
- Ferris, F. L., Kassoff, A., Bresnick, G. H., & Bailey, I. (1982). New visual acuity charts for clinical research. American Journal of Ophthalmology, 94(1), 91–96.
- Elliott, D. B., Yang, K. C. H., & Whitaker, D. (1995). Visual acuity changes throughout adulthood in normal, healthy eyes: Seeing beyond 6/6. Optometry and Vision Science, 72(3), 186–191.
- World Health Organization. (2019). International Classification of Diseases, 11th Revision (ICD-11). WHO.
- Directive 2006/126/EC of the European Parliament and of the Council of 20 December 2006 on driving licences (recast). Official Journal of the European Union, L 403, 18–60.
- Commission Directive 2009/113/EC of 25 August 2009 amending Directive 2006/126/EC on driving licences. Official Journal of the European Union, L 223, 31–35.
Frequently asked questions
Can I test my eyesight online accurately?
Roughly. With a calibrated screen and a measured distance, an online Tumbling E test gives an approximate acuity. It cannot measure a glasses prescription or find eye diseases, so it does not replace an eye exam.
What does 20/20 vision mean?
It means you can read at 20 feet a letter that is 5 minutes of arc tall, the standard Herman Snellen set in 1862. In metres it is written 6/6 and in decimal 1.0. Many healthy adults see better than 20/20.
Why do I need a bank card?
The test must know how big a millimetre is on your screen to draw letters at the right size. A standard bank card is 85.6 × 53.98 mm on every card, so it works as a ruler.
Which distance should I choose?
The farthest one you can use reliably. 2 m or 3 m is closest to a real distance chart. At 50 cm, people over about 40 may score lower because near focus weakens with age.
Can I use this test for my child?
No. Children need a professional eye test, because a lazy eye or a squint can be missed and needs early treatment.
Does this test tell me my glasses prescription?
No. Acuity tells you how small a letter you can read, not which lens corrects it. Only a refraction by an optometrist or ophthalmologist gives a prescription.
What is a Tumbling E?
It is the letter E turned in one of four directions. You say which way its bars point, so the test works without reading any alphabet.