A Snellen chart is the familiar eye chart with rows of letters that get progressively smaller from top to bottom, used to measure how sharp your distance vision is. Designed by Dutch ophthalmologist Hermann Snellen in 1862, it remains the most widely used tool for testing visual acuity in eye exams worldwide. The large “E” at the top and the increasingly tiny lines below it are something most people recognize, even if they’ve never known the chart’s name.
How the Chart Works
You stand 20 feet from the chart (or 6 meters in countries using the metric system) and read the smallest line of letters you can see clearly. Each row corresponds to a specific level of visual sharpness. The letters on the chart, called optotypes, are designed with precise proportions: the standard letter “E” spans five minutes of visual angle at its intended viewing distance, with each arm of the letter and each gap between the arms occupying exactly one minute of arc. That level of geometric precision is what makes the test standardized rather than arbitrary.
Before Snellen created his chart, every eye doctor used their own preferred reading material to check patients’ sight. There was no common standard, which made it impossible to compare results between doctors or track a patient’s vision over time. Snellen’s colleague Fransiscus Donders had been asking patients to read from wall charts during exams and recognized the need for something uniform. He asked Snellen to design one, and the result became the global default.
What the Numbers Mean
Your result is written as a fraction like 20/20, 20/40, or 20/100. The top number is your distance from the chart, which is almost always 20 feet. The bottom number is the distance at which someone with normal vision could read that same line. So if your score is 20/40, you need to be 20 feet away to read what a person with normal eyesight can read from 40 feet. The larger the bottom number, the worse your visual acuity.
A score of 20/20 is considered normal, not perfect. Some people see better than that. A result of 20/15 means you can read at 20 feet what most people need to be 15 feet away to see. On the other end of the spectrum, 20/200 is the threshold for legal blindness in the United States. The Social Security Administration defines this as central visual acuity of 20/200 or less in your better eye, even with corrective lenses. A person can also meet the legal definition of blindness through severe visual field loss, where the widest diameter of their remaining visual field is 20 degrees or less.
Common Visual Acuity Scores
- 20/15 or 20/10: Better than average sharpness.
- 20/20: Normal visual acuity. You can resolve fine detail at the expected distance.
- 20/40: The minimum standard for an unrestricted driver’s license in most U.S. states. Objects need to be about twice as close for you to see them as clearly as someone with 20/20 vision.
- 20/70 to 20/100: Moderate visual impairment. Reading signs, recognizing faces at a distance, and driving become significantly harder.
- 20/200 or worse: Legally blind with best correction. At this level, the largest letter on many Snellen charts is difficult to make out.
What the Test Does and Doesn’t Tell You
A Snellen chart measures one specific thing: your ability to distinguish fine detail at a distance. It does not check peripheral vision, depth perception, color vision, or how well your eyes focus up close. It also doesn’t diagnose the cause of poor vision. If your score comes back worse than expected, your eye care provider will use additional tests to figure out whether the issue is a refractive error like nearsightedness or astigmatism, a problem with the lens such as cataracts, or something affecting the retina or optic nerve.
The chart is typically used twice during an exam: once without correction to get your baseline acuity, and again with glasses or contact lenses (or through a device called a phoropter) to determine your best-corrected vision. The difference between those two numbers tells a lot. If correction brings you to 20/20, the problem is optical and fixable with lenses. If it doesn’t, something else is going on.
Versions for Children and Non-Readers
The standard Snellen chart assumes you know the English alphabet, which obviously excludes young children and anyone unfamiliar with Latin letters. Several alternatives exist for these situations. The Tumbling E chart uses a single letter “E” rotated in different directions: up, down, left, and right. You point or say which way the arms of the E face. A Landolt C chart works similarly, showing a ring with a gap that you identify by its position. Both approaches test the same visual ability without requiring literacy.
For preschoolers, eye care providers often use LEA Symbols or HOTV letters. LEA Symbols replace the alphabet with simple shapes that young children can name or match to a card they hold. HOTV charts use just four easily distinguishable letters, and children can point to matching letters on a response card rather than reading aloud. The American Association for Pediatric Ophthalmology and Strabismus recommends these tools for children who don’t yet know their letters.
Why It Still Dominates
The Snellen chart has known limitations. The number of letters per line varies (the top rows have fewer), which makes some lines statistically easier to read. The spacing between letters also changes from row to row, and these inconsistencies can affect results. A more modern alternative called the LogMAR chart (often in the ETDRS format) fixes these issues by using the same number of letters per line and consistent spacing. Research settings and clinical trials almost always use LogMAR charts because they produce more precise, repeatable measurements.
Yet Snellen charts persist in everyday clinical practice because they’re fast, cheap, and familiar. They take under a minute to administer, require no special equipment beyond the printed chart and proper lighting, and give results in a fraction that patients and providers instantly understand. For routine screening, that combination is hard to beat, even 160 years after Hermann Snellen first put letters on a wall.