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Vision Test Online
This vision test online is a free Snellen-style eye chart that runs on your screen. You calibrate it with a credit card, hold your device at the stated distance, and read the lines from largest to smallest. Tap the smallest line you can read clearly, and the test gives you an approximate visual acuity — from 20/200 down to 20/20 — with a plain-English note about what it means and when to book a proper eye exam.
It takes about a minute, and nothing is recorded or sent anywhere — the whole test runs in your browser. A Snellen chart is the chart with the big "E" at the top that eye doctors have used for over 150 years. It measures how small a letter you can read at a set distance. This online version does the same in a simplified way: it cannot replace an eye exam, but it can tell you whether your distance vision is roughly normal or time to get checked properly.
Step 1 — calibrate: hold a credit card (85.6 × 54 mm) against the blue box below and move the slider until the box matches the card exactly. Step 2 — distance: hold or place your device at the test distance. Step 3: cover one eye, read down the chart, and tap the smallest line you can read clearly.
How it works
A Snellen chart measures visual acuity — the smallest detail your eye can resolve at a set distance. Each line of the chart is sized so that its letters subtend a known angle at your eye. The letters on the 20/20 line are exactly 5 arc-minutes tall at the test distance, which is the smallest detail a healthy eye can normally resolve. Bigger lines are multiples of that: the 20/40 line is twice as tall, the 20/100 line is five times as tall, and the 20/200 line is ten times as tall.
What the numbers mean. Visual acuity is written as a fraction, such as 20/25. The first number is the test distance in feet — 20 feet is the standard — and the second number is the distance at which a person with normal vision could read the same line. So 20/25 means you can read at 20 feet what a person with normal vision could read at 25 feet: slightly below average, but close. 20/20 is normal distance vision. 20/40 means you can read at 20 feet what a normal eye reads at 40 feet — in many places, 20/40 is the minimum for driving without glasses. 20/200 is the threshold for legal blindness in many countries when it cannot be corrected with glasses.
How this online version is calibrated. A chart only works if the letters are the right physical size on your screen, and screens differ wildly. This test handles that in two steps. First, you tell it your screen size and test distance, and it works out the letter heights from the formula above. Second, you calibrate with a real credit card against the blue box, which corrects for your screen's pixel density and browser zoom. Without calibration, the result is only a rough guess.
How to take the test properly. Sit or stand at the test distance — 10 feet (3 metres) is the default, and 20 feet (6 metres) is the classic standard if you have the room. Good lighting matters: the screen should be bright, and there should be no glare or reflections on it. Cover one eye without pressing on it and read down the chart. Do not squint — it artificially sharpens the image. Stop at the first line where you cannot read every letter, tap it, then repeat with the other eye.
Worked example
Here is a worked example, so you can see exactly what the result looks like.
Step 1: James calibrates the chart with his credit card, sets the distance to 10 feet, and covers his left eye.
Step 2: He reads the 20/200, 20/100, 20/70, 20/50, 20/40 and 20/30 lines without trouble, and then the 20/25 line — he reads every letter on it clearly.
Step 3: He moves to the 20/20 line, the smallest on the chart, and finds he can only guess at the letters — they blur together and he cannot read them clearly.
Step 4: He taps the 20/25 line, the smallest line he could read clearly, and repeats the test with his other eye.
Try it yourself: the tool gives the same result for that run — About 20/25 — with the note that his distance vision is slightly below the 20/20 standard but close to normal, and that an eye exam is worth booking if it is worse than 20/40 or if it has changed recently.
FAQ
Is an online vision test accurate?
It is a useful screening check, not a diagnosis. If the chart is calibrated properly with a credit card and you sit at the stated distance, it gives a fair estimate of your distance vision. Use it to decide whether to book a proper eye exam.
Why do I need to calibrate with a credit card?
Because every screen is different. The letters on a Snellen chart must be an exact physical size — at 10 feet, the 20/20 letters are only 4.4 mm tall. Matching the blue box to a real card tells the test exactly how big a millimetre is on your screen.
What does 20/20 vision mean?
20/20 is normal distance vision: you can read at 20 feet what a person with normal vision can read at 20 feet. It does not mean perfect vision — it is a measure of sharpness at distance only. Plenty of people with 20/20 vision still need reading glasses, and many people see better than 20/20, such as 20/15.
What if one eye is much worse than the other?
That is common, and it is one reason to test each eye separately. If one eye reads two or more lines worse than the other — for example 20/20 in one eye and 20/50 in the other — book an eye exam. A big difference can also affect depth perception.
How often should I have my eyes tested?
Most adults should have a full eye exam every two years, and every year if you are over 60, have diabetes, have a family history of eye disease, or already wear glasses. An online test is a handy check between exams, but it is not a substitute for one.