π 20/20 Vision Calculator
Convert Snellen, decimal, and LogMAR vision measurements.
Decimal visual acuity
For informational purposes only, not medical advice. Consult a healthcare professional.
"20/20 vision" does not mean perfect vision. It means you see at 20 feet what a person with standard vision sees at 20 feet. Someone with 20/40 vision needs to stand 20 feet from an object to see what a 20/20 person sees from 40 feet away. The first number is always your testing distance; the second is the distance at which the letters you read look normal-sized to standard eyes.
This calculator converts any Snellen fraction into its decimal and LogMAR equivalents, the two scales eye researchers and low-vision specialists actually use. Enter the smallest line you could read on the chart and you get all three numbers plus what the result means in plain language.
How to use this calculator
Pick the smallest line you read clearly on a standard eye chart, with your usual glasses or contacts on if you wear them. The calculator does the rest instantly.
Choose your Snellen line from the dropdown, for example 20/40 if that was the smallest row you read without mistakes.
Read your decimal acuity (20/20 becomes 1.00, 20/40 becomes 0.50) and your LogMAR value (0.00 is standard, higher is worse).
Check the category. Anything worse than 20/40 deserves an eye exam if you have not had one in the last year or two.
The three scales, explained
Eye care uses three ways to write the same measurement. Snellen fractions (20/40) are what patients see on the chart. Decimal acuity divides the top number by the bottom, so 20/40 is 0.50 and 20/200 is 0.10. LogMAR takes the negative log of the decimal value, which gives researchers a scale where each chart line is an equal step: 0.0 is 20/20, 0.3 is 20/40, 1.0 is 20/200.
LogMAR looks intimidating but it is the fairest scale, because going from 20/200 to 20/100 is a much bigger real-world change than going from 20/25 to 20/20, and LogMAR reflects that. Clinical trials report vision in LogMAR for exactly this reason. A handy anchor: 20/40 vision, the cutoff for an unrestricted driver license in most US states, is 0.50 decimal and 0.30 LogMAR.
Worked example
Say your exam card reads 20/40 in your right eye. Decimal acuity is 20 divided by 40, which is 0.50. LogMAR is the negative log of 0.50, which is 0.30. The calculator labels this normal vision, and it clears the driver license standard in most states, though you might still want glasses for night driving or small print.
Now take 20/200, the US legal blindness threshold. Decimal acuity is 20 divided by 200, or 0.10. LogMAR is 1.00. That is a full 1.0 LogMAR worse than standard vision, ten equal chart steps down, which shows why LogMAR is useful: it turns the chart into a ruler instead of a guessing game.
What your result means
Vision of 20/40 or better is functionally normal for daily life. Between 20/50 and 20/60, you are in mild impairment territory: fine indoors, squinting at highway signs. Worse than 20/60 counts as moderate impairment by World Health Organization standards, and 20/200 or worse in the better-seeing eye with best correction is the US legal blindness threshold, which qualifies people for services and benefits rather than describing total darkness.
Two things these numbers do not capture. First, they measure only sharpness straight ahead, not peripheral vision, depth perception, or night vision. Second, they describe your vision with correction during the test. If your number dropped since your last exam, the most likely explanation is an outdated prescription, followed by dry eye or cataract as people age. Either way, the next step is an exam, not worry.
When to get your eyes checked
Adults with no symptoms or risk factors should get a baseline full eye exam around age 40, then follow their eye doctor's schedule. Get checked sooner if your vision changes, you see flashes or new floaters, or you have diabetes, which needs yearly dilated exams. Kids should be screened at well-child visits, since uncorrected vision problems can masquerade as learning trouble.
For overall wellness habits that support eye health too, our water intake calculator covers hydration, and the sleep cycle calculator helps you plan rest that reduces eye strain and fatigue.
Why two people with 20/40 see differently
Snellen acuity is one dimension of vision, and real-world sight has several. Two people can both test 20/40 and describe very different experiences. One might have simple uncorrected nearsightedness that glasses fix completely; the other might have early cataract, where the chart is readable but everything looks hazy, or astigmatism, where letters smear in one direction.
Contrast sensitivity is the biggest missing piece. The chart uses black letters on a bright white background, maximum contrast. Real life is gray on gray: faces in dim restaurants, road signs in rain. Someone with 20/25 acuity but poor contrast sensitivity can struggle more day to day than someone with 20/40 and crisp contrast. Glare sensitivity works the same way, which is why night driving bothers people whose daytime chart looks fine.
The practical takeaway: if your chart number and your lived experience disagree, say so at the exam. "I read 20/30 but I cannot see at night" points the doctor toward contrast and glare testing, refraction under different lighting, or a cataract check. The Snellen line is the start of the conversation, not the whole of it.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What does 20/20 vision actually mean?
It means you see at 20 feet what a person with standard vision sees at 20 feet. It is normal sharpness, not perfect vision: it says nothing about peripheral vision, color vision, depth perception, or eye health.
Is 20/40 vision bad?
No. 20/40 is still in the normal range and meets the driver license standard in most US states. It equals 0.50 decimal or 0.30 LogMAR. You might notice small print or distant signs are slightly softer, and glasses can sharpen it.
What is the difference between 20/20 and 20/15?
20/15 is sharper than average: you see at 20 feet what standard eyes need 15 feet to see. It converts to 1.33 decimal and about -0.12 LogMAR. Some people naturally test here, especially younger adults.
What does LogMAR 0.0 mean?
LogMAR 0.0 equals 20/20 vision exactly. Each 0.1 step is one line on the chart, so LogMAR 0.3 is 20/40 and LogMAR 1.0 is 20/200. Researchers prefer it because every step represents an equal change in vision.
What prescription corresponds to 20/200?
There is no fixed conversion between Snellen acuity and glasses prescription. Two people with 20/200 can need very different lenses, and some causes of 20/200 (like cataract or retinal disease) are not fixable with glasses at all.
Can 20/20 vision still need glasses?
Yes. The Snellen chart only tests distance sharpness. You can read the 20/20 line and still need glasses for astigmatism symptoms, reading up close (presbyopia), or eye strain, and you can have eye disease with 20/20 vision.
What is considered legally blind in the US?
Visual acuity of 20/200 or worse in the better-seeing eye with best correction, or a visual field narrower than 20 degrees. It is a legal threshold for benefits and services, not a medical description of total blindness.
How often should I get an eye exam?
Adults with no symptoms or risk factors need a baseline full eye exam around age 40. People with diabetes need yearly dilated exams, and anyone noticing vision changes should go promptly rather than waiting for a schedule.