Using a Snellen chart correctly comes down to three things: standing exactly 20 feet (6 meters) from the chart, testing one eye at a time, and reading the smallest line you can see clearly. Whether you’re administering the test for someone else or checking your own vision at home, the process is straightforward once you know the setup.
Setting Up the Chart
The standard testing distance is 20 feet, or about 6 meters. This distance is built into how the chart works. Every score you get assumes you were standing exactly that far away, so getting it wrong throws off the results. If you’re in a clinic, there’s usually a mark on the floor. At home, measure the distance with a tape measure rather than estimating.
Hang the chart at eye level on a well-lit wall. Lighting on the chart itself matters more than the overall brightness of the room. You want even, bright illumination across the letters with no glare or shadows. A desk lamp angled toward the chart works in a pinch, but avoid positioning it where it reflects light back into the reader’s eyes.
If you’re testing in a smaller room, some charts are designed for shorter distances (like 10 feet), with letter sizes adjusted accordingly. If you use one of these, record the actual testing distance, because it changes how you interpret the score.
Step-by-Step Testing Process
Start by deciding whether you’re testing corrected or uncorrected vision. For uncorrected acuity, remove glasses or contact lenses. For corrected acuity, keep them on. Most driving and workplace screenings test corrected vision, meaning you wear whatever prescription you normally use.
Here’s the sequence:
- Cover one eye. Use an opaque card, a plastic occluder, or even a cupped hand. The key is blocking vision completely without pressing on the closed eye, which can temporarily blur it when you switch sides.
- Read from the top down. Start with the large letter at the top of the chart and work your way to smaller lines. Read each letter aloud (or have the person being tested read aloud to you).
- Stop at the smallest readable line. If all five letters on a line are correct and there are more lines below, keep going. The last line where most letters are read correctly is the score for that eye.
- Switch eyes. Cover the other eye and repeat the entire process.
Test the right eye first by convention, though the order doesn’t affect results. If someone squints or tilts their head, note it. Squinting artificially sharpens focus by narrowing the light entering the eye, which can make the score look better than it really is.
How to Read the Score
Each line on the chart has a fraction printed next to it, like 20/20, 20/40, or 20/200. The top number is your testing distance in feet (almost always 20). The bottom number is the distance at which a person with normal vision could read that same line.
So if your result is 20/40, it means you need to stand 20 feet away to read what someone with normal vision could read from 40 feet. The higher the bottom number, the worse the acuity. A score of 20/20 is considered normal. A score of 20/200, where you can only read the giant top letter, is the threshold for legal blindness in the United States when it can’t be corrected with lenses.
If someone reads most but not all letters on a line, you can note partial credit. For example, “20/30 minus 2” means they reached the 20/30 line but missed two letters. This level of detail matters more in clinical settings than in casual home checks.
What the Scores Mean in Practice
The score that comes up most in daily life is the driving requirement. Nearly every U.S. state requires corrected visual acuity of at least 20/40 in the better eye to hold a standard driver’s license. A few states are slightly more lenient: Georgia allows 20/60, and New Jersey and Wyoming set the cutoff at 20/50.
Here’s a rough guide to what common scores feel like:
- 20/20: Normal acuity. You can read standard text, road signs, and most everyday print without difficulty.
- 20/40: The minimum for driving in most states. You can function well but may struggle with smaller print or distant signs, especially at night.
- 20/70 to 20/100: Noticeably blurred distance vision. Reading a menu board from across a room or recognizing faces at a distance becomes difficult.
- 20/200 or worse: Only the largest letters or objects are visible. This level of impairment significantly affects independence without corrective lenses or aids.
It’s also possible to score better than 20/20. A result of 20/15 means you can see at 20 feet what most people need to be 15 feet away to read. This is common in younger adults and simply reflects sharper-than-average acuity.
What the Snellen Chart Doesn’t Test
The chart measures one thing: how well you resolve high-contrast black letters on a white background at a fixed distance. That’s useful, but it misses several aspects of vision that affect daily life.
It won’t detect astigmatism on its own, though astigmatism will lower your score. It doesn’t measure contrast sensitivity, which is your ability to distinguish objects that are close in brightness to their background (like seeing a gray car on a gray road in fog). It doesn’t check peripheral vision, depth perception, or color vision. And a standard wall chart tests distance vision only. Near vision, the kind you use for reading or phone screens, requires a separate card held at about 16 inches (40 cm).
A perfect 20/20 score also doesn’t rule out eye disease. Conditions like glaucoma can destroy peripheral vision while leaving central acuity intact for years. The Snellen chart is a starting point, not a comprehensive eye exam.
Tips for Accurate Results at Home
Printable Snellen charts are widely available online, but accuracy depends on printing them at the correct size. The letters on the 20/20 line should be about 8.75 millimeters tall. If you scale the print to fit a different paper size, the measurements are off and so is every score you take. Look for charts that specify “print at 100%” and verify the size with a ruler before testing.
Test in consistent conditions each time. Same lighting, same distance, same chart. This way, if your score changes over weeks or months, you’ll know the difference is in your eyes and not your setup. And don’t memorize the chart. If you’ve seen it enough times to recite the letters, switch to a different version or have someone point to individual letters at random.
Home testing is useful for monitoring trends, like noticing that one eye has gotten gradually worse, but it’s not a substitute for a professional exam with proper equipment. If your home scores consistently fall below 20/40, or if one eye is noticeably worse than the other, that’s worth following up on.