A Snellen chart measures how sharply you can see by having you read rows of letters that shrink line by line from top to bottom. The result is written as a fraction like 20/20 or 20/40, and understanding what that fraction means is simpler than it looks. Here’s how the whole process works.
How the Test Is Set Up
The chart hangs on a wall, and you stand or sit exactly 20 feet (6 meters) away from it. That distance is standardized so results are comparable from one test to the next. If the room is too short for 20 feet, some offices use a mirror to simulate the correct distance or a smaller chart calibrated for a shorter viewing range.
Your provider will ask you to cover one eye, usually with your hand or a small paddle, so each eye is tested independently. You start with the largest letter at the top and read downward, row by row, calling out each letter until the characters are too small to identify clearly. Then you switch eyes and repeat.
If you wear glasses or contacts, you’ll typically be tested both with and without them. The uncorrected reading shows your natural vision, while the corrected reading shows how well your lenses are working.
What the Numbers Mean
Your result is recorded as a fraction. The top number (numerator) is the distance you stood from the chart, which is almost always 20 feet. The bottom number (denominator) is the distance at which a person with normal vision could read the same line you just barely managed to read.
So 20/20 means you read at 20 feet what a person with normal sight reads at 20 feet. That’s the baseline for “normal” visual acuity. If your result is 20/40, it means the smallest line you could read is one that a person with normal vision could read from 40 feet away. In other words, you need to be closer to see what others see from farther back.
A result like 20/15 is actually better than average. It means you can read at 20 feet what most people need to be 15 feet away to see. The smaller the bottom number, the sharper your vision. The larger the bottom number, the more impaired it is.
Common Results and What They Indicate
- 20/20: Normal visual acuity. This is the standard benchmark.
- 20/40: The minimum acuity required for an unrestricted driver’s license in many U.S. states.
- 20/70 to 20/160: Considered low vision. Daily tasks like reading signs or recognizing faces at a distance become difficult.
- 20/200 or worse: The threshold for legal blindness in the United States, as defined by the Social Security Administration, when measured in your better eye with the best available correction.
How the Letters Are Designed
The letters on a Snellen chart aren’t pulled from a standard font. Dutch ophthalmologist Herman Snellen designed them in the 1860s on a 5-by-5 grid, where every stroke of the letter is exactly one-fifth the height of the whole character. This keeps recognition dependent on the sharpness of your vision rather than on any quirks of letter shape. A normal eye can distinguish these strokes when they span an angle of five arc-minutes, which is the optical basis for the entire chart’s scaling.
Each row is sized so that a person with 20/20 vision can read it from a specific distance. The giant “E” at the top of most charts corresponds to 20/200. As you move down, the letters shrink in precise steps, with each line labeled with its fraction on the side of the chart.
Charts for Children and Non-Readers
If you can’t read the Latin alphabet, whether because of age or language, the Tumbling E chart is the most common alternative. It uses a single capital letter E rotated in four directions: up, down, left, and right. Instead of calling out letters, you point your fingers in the direction the “arms” of the E are facing. The scoring works identically to the standard Snellen chart.
Other variants use pictures (like a house, star, or apple) for very young children. These symbol charts are less precise than letter-based tests but are useful for screening kids who don’t yet know their letters.
How to Tell Which Line You “Passed”
Your acuity score corresponds to the smallest line where you correctly identified most of the letters. You don’t need to get every single letter on a line right. Providers typically record the line along with how many letters you missed, written as something like “20/30 -2,” meaning you read the 20/30 line but missed two letters on it.
If you can only read the top letter, your acuity is recorded as 20/200. If you can’t even see that, the provider may move you closer to the chart and adjust the fraction accordingly, or switch to other methods like counting fingers or detecting hand motion.
What the Chart Does Not Measure
The Snellen chart tests one specific thing: your ability to recognize small, high-contrast black letters on a bright white background under good lighting. That’s a narrow slice of how vision works in everyday life.
It doesn’t measure contrast sensitivity, which is your ability to distinguish objects that are only slightly different from their background. Missing the last step on a staircase or not spotting a curb between the road and the sidewalk has nothing to do with how small an object is. It’s about subtle differences in shading and color. Someone can score 20/20 on a Snellen chart and still struggle with low-contrast situations, especially in dim lighting.
The chart also doesn’t test peripheral vision, depth perception, color vision, or how well your eyes work together. A full eye exam includes several additional tests to evaluate those areas. The Snellen chart is a starting point, not a complete picture of your visual health.