It is a ratio of distances, not a percentage or a score. The American Academy of Ophthalmology explains that the top number is how far in feet you stand from the chart, and the bottom number is the distance at which a person with normal sight reads the same line you just read. Matching numbers mean your distance vision matches the reference, and nothing more.

MedlinePlus states it the other way: 20/40 means the line you read at 20 feet is one an average eye reads from 40 feet. So the bottom number is how far back an average eye could have stood - three times your distance at 20/60, ten times at 20/200. Acuity measures one narrow thing: how fine a detail you resolve at distance.

Hermann Snellen, a Dutch eye doctor, developed the chart in the 1860s at the request of his colleague Franciscus Donders. Twenty feet is close enough to optical infinity that the eye relaxes instead of focusing inward. MedlinePlus gives the distance as 20 feet, or 6 metres - the entire explanation for 6/6, the notation a clinic outside the United States writes. 6/6 is 20/20 and 6/12 is 20/40. Short rooms use a mirror, with the same results.

Here 20/20 stops being folklore and becomes regulation. Under 49 CFR 391.41(b)(10), a United States commercial driver needs distant acuity of at least 20/40 in each eye with or without lenses, binocular acuity of at least 20/40, a field of at least 70 degrees in the horizontal meridian in each eye, and the ability to recognise standard red, green and amber traffic signals.

Aviation is stricter. 14 CFR 67.103 requires a first-class airman medical certificate applicant to show distant acuity of 20/20 or better in each eye separately, plus near vision of 20/40 at 16 inches; applicants aged 50 or older must also meet 20/40 at 32 inches. Those rules are the only places where 20/20 is a pass mark rather than a description.

No, and that is the usual misreading. 20/20 is normal - a mid-range reference point, not a ceiling, since plenty of people measure 20/15 or 20/10. It also says nothing about colour vision, depth perception, peripheral field, contrast sensitivity or eye pressure, which is why an exam measures several things and a chart measures one.

The World Health Organization reports at least 2.2 billion people with a near or distance vision impairment, and at least 1 billion cases that were preventable or remain unaddressed: presbyopia 826 million, cataract 94 million, uncorrected refractive error 88.4 million. The National Eye Institute puts refractive errors at more than 150 million Americans, many unaware they could see better.

It is a game, and its listing says so. Sight Check: Eye Chart Game shows rows of shrinking letters you read aloud from across the room, saves your score against a personal best, and keeps results on the device. It is free, 54.8 MB, rated 12+, version 1.0 from 15 July 2026, ad-free, and listed in English only. The listing is also the honest limitation, in the developer's own words on the App Store:

Take that as the scope statement it is. It can show you that the fifth row blurred this year when it did not last year - a useful prompt to book an appointment. It cannot tell you your refraction.

Three situations. First, if anything changed suddenly - vision lost in one eye, new flashes or floaters, pain, double vision - because acuity apps measure a steady state and are blind to an acute event. Second, if you need a number that counts: the 20/40 driving floor and the 20/20 airman standard are measured by an examiner under controlled conditions. Third, for a child, where an uncalibrated screen at an uncontrolled distance yields a number that means little.

With that understood, Sight Check: Eye Chart Game is a reasonable way to watch your own distance vision between appointments - on the App Store and on Google Play as Sight check game, free, with results staying on your phone.