Snellen Eye Chart

The Snellen eye chart is the standard tool used by eye care professionals worldwide to measure visual acuity. Developed by Dutch ophthalmologist Herman Snellen in 1862, it consists of rows of letters that decrease in size. The chart is designed to be viewed from 20 feet (6 meters), and each row is labeled with a fraction like 20/20, 20/40, etc. The numerator (top number) represents the testing distance, and the denominator (bottom number) indicates the distance at which a person with normal vision can read that row.

20/20 vision is considered "normal" visual acuity. It means that at 20 feet, you can clearly see what a person with normal vision should be able to see at 20 feet. If you have 20/40 vision, it means you need to be at 20 feet to see what a person with normal vision can see at 40 feet — your vision is worse than average. Conversely, 20/15 vision means you can see at 20 feet what most people can only see at 15 feet — your vision is better than average.

Online vision tests can provide a rough screening estimate of your visual acuity, but they have significant limitations. Accuracy depends on: screen size and resolution, proper calibration, ambient lighting, viewing distance, and screen glare. This tool uses credit card calibration to improve accuracy, but it cannot replace a comprehensive eye exam by an optometrist or ophthalmologist. Professional exams check for eye diseases, astigmatism, color blindness, depth perception, and other conditions that online tests cannot detect.

Click the "Calibrate Screen" button above. A dashed red rectangle will appear. Take a standard credit card (85.6 mm / 3.37 inches wide — all bank cards, driver's licenses, and ID cards use this ISO standard size) and hold it against your screen. Adjust the slider until the rectangle exactly matches the width of your card. This calibrates the tool to your screen's pixel density, ensuring letters are displayed at the correct size for your viewing distance. Once calibrated, the setting is saved in your browser.

The standard Snellen test distance is 20 feet (6 meters). However, for practical home use, a distance of 2–3 meters (6.5–10 feet) from a desktop monitor works reasonably well. The tool automatically adjusts letter sizes based on your selected distance. For the most accurate results, measure your actual viewing distance with a tape measure and select "Custom" to enter the exact value. Ensure the room is well-lit and your screen brightness is comfortable.

Snellen Score	Decimal	Category	Notes
20/10 – 20/15	2.0 – 1.33	Excellent	Better than average vision
20/20	1.0	Normal	Standard normal vision
20/25 – 20/30	0.8 – 0.63	Mild impairment	Usually not requiring correction
20/40 – 20/70	0.5 – 0.3	Moderate impairment	May need glasses; 20/40 is the driving limit in most US states
20/100 – 20/200	0.2 – 0.1	Significant impairment	Legal blindness threshold (20/200 or worse with best correction)

You should schedule a professional eye exam if: (1) your visual acuity is worse than 20/40 in either eye, (2) you experience frequent headaches, eye strain, or blurred vision, (3) you notice sudden changes in your vision, (4) you see floaters, flashes, or dark spots, (5) you have a family history of eye disease, or (6) it has been more than 2 years since your last eye exam. Adults aged 18–60 should have an eye exam every 1–2 years; children and seniors should be checked annually.

Randomizing the letters prevents memorization bias. In a real eye exam, the doctor may switch between different versions of the chart so you cannot simply recall the sequence from memory. Our refresh feature randomly selects from the standard Snellen letter set (C, D, E, F, L, N, O, P, T, Z) for each row, ensuring each test is unique and your results are more reliable.