An eye chart (often written as “i chart”) is a poster of letters in decreasing sizes used to measure how sharp your vision is. The most common version is the Snellen chart, invented in 1862 by Dutch ophthalmologist Herman Snellen. You’ve almost certainly seen one: rows of black letters on a white background, with one giant letter at the top and progressively smaller lines below. It remains the standard tool for testing distance vision in eye exams worldwide.
How the Chart Measures Your Vision
During an eye exam, you stand 20 feet (6 meters) from the chart and read the smallest line of letters you can see clearly. Each line corresponds to a fraction like 20/20, 20/40, or 20/100. The top number is always your testing distance (20 feet). 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 be 20 feet away to read what someone with normal sight can read from 40 feet. If your result is 20/100, you must stand at 20 feet to see what a normally sighted person sees from 100 feet away. The smaller that bottom number, the sharper your vision. A result of 20/20 is considered normal distance vision.
Countries using the metric system express the same measurement at 6 meters instead of 20 feet. A score of 6/6 is the metric equivalent of 20/20, and 15/15 (tested at 15 feet) means the same thing. The ratio matters, not the raw numbers.
What Makes 20/20 the Standard
The 20/20 line is based on the smallest detail the human eye can resolve: one arc minute, which is 1/60th of a single degree of your visual field. Early vision scientists determined this was the tightest angle at which a person could distinguish two separate points. At 20 feet, a letter designed to hit that threshold is about 8.75 millimeters tall. Every eye chart, regardless of design or brand, calibrates its 20/20 line to this exact angular size.
The letter sizes scale geometrically. Letters on the 20/40 line are exactly twice the size of those on the 20/20 line. Letters on the 20/200 line (the big letter at the top) are ten times larger. This consistent scaling is what makes the chart a reliable measuring tool rather than just a random collection of letters.
The Letters on the Chart
A standard Snellen chart uses only nine letters: C, D, E, F, L, O, P, T, and Z. These are called optotypes, and they were chosen because their features (straight lines, curves, and gaps) test specific aspects of visual resolution. Each letter fits within a grid where the overall height is five times the thickness of its individual strokes, keeping the geometry consistent across every line.
Not every eye chart uses these same letters. In countries where the writing system isn’t based on the Latin alphabet, or when testing young children and people who can’t read, alternatives are common. The Tumbling E chart shows the letter E rotated in four directions, and you point the direction the “legs” face. The Landolt C chart uses a ring with a gap, and you identify where the opening is. Both are widely used in pediatric eye exams, with studies confirming they work reliably in children younger than 10.
The ETDRS Chart: A More Precise Version
While the Snellen chart works well in a routine eye exam, it has some design flaws that matter in clinical research. The number of letters per line isn’t consistent, the spacing between letters varies, and repeat tests on the same person can swing by as many as 5 to 16.5 letters, making it hard to detect small changes over time.
The ETDRS chart was developed to fix these problems. It uses five letters on every line, consistent spacing, and a scoring system called LogMAR that assigns a precise numerical value to each letter read correctly. Test-retest variability drops to 3.5 to 10 letters, roughly half the inconsistency of a Snellen chart. The U.S. Food and Drug Administration requires ETDRS-based testing for clinical trials involving vision, making it the gold standard in research settings. LogMAR itself isn’t a chart type but a scoring notation where lower numbers mean better vision, with each letter worth 0.02 units and each full line worth 0.1.
What Your Results Mean in Practice
A 20/20 result means your distance vision is normal. It does not mean your vision is perfect. You can still have issues with near vision, peripheral vision, depth perception, color vision, or eye health that a chart alone won’t detect.
Results worse than 20/20 suggest you may benefit from glasses or contact lenses. A few key thresholds are worth knowing:
- 20/40: The minimum standard for an unrestricted driver’s license in most U.S. states.
- 20/70 to 20/160: Considered low vision, where corrective lenses may not fully restore normal acuity.
- 20/200 or worse (with best correction): The threshold for statutory blindness in the United States. The Social Security Administration defines this as being unable to read any letters on the 20/100 line of a standardized chart, even with glasses or contacts.
Why Lighting and Distance Matter
An eye chart gives accurate results only under the right conditions. The standard testing distance is 20 feet. If the room is too short, a mirror is sometimes used to simulate that distance, or a chart calibrated for a shorter distance (often 10 feet) is substituted.
Lighting plays a surprisingly large role. Research on illumination and acuity testing found that keeping the chart lit between 400 and 600 lux limits measurement error to a negligible amount (about one hundredth of a LogMAR line). Clinical research protocols for ETDRS charts call for even brighter conditions, between 807 and 1,345 lux. A dimly lit hallway or an overly bright window behind you can shift your results.
Testing Your Vision at Home
Printable eye charts are available from sources like university ophthalmology departments, but they require careful setup to produce meaningful results. After printing on standard letter-size paper (8.5 by 11 inches), you need to verify the scale is correct. Most printable charts include an image of a U.S. quarter that should measure exactly one inch across when printed at 100% size. If the quarter is too large or too small, your letter sizes will be off and the results unreliable.
For distance testing, position the chart on a wall at eye level in a well-lit room, then stand exactly 20 feet away. Cover one eye completely and read the smallest line you can. Repeat with the other eye. For near vision charts, the typical testing distance is 14 inches, which you can approximate using the diagonal of a standard sheet of paper.
Home testing is useful for spotting noticeable changes in your vision between professional exams, but it can’t replace a full evaluation. Without controlled lighting, precise distance measurement, and a clinician interpreting the results alongside other tests, a home chart gives you a rough estimate rather than a clinical measurement.