Visual Acuity Test

Visual acuity is the measure of how small a detail your eye can make out at a given distance. It is the first thing an optometrist or ophthalmologist checks, usually with a letter chart on the wall. This online version uses the tumbling E: a single letter E that points up, down, left or right. You simply tell the test which way the open side of the E is facing, so you do not need to know the alphabet, and children who can point can take part too.

Before you start, you calibrate the screen by resizing an on-screen box until it matches a standard bank card. This lets the site draw each E at the correct physical size for the distance you choose (3 metres by default). You test the right and left eye separately, and the E gets smaller after each correct answer. At the end you get an approximate Snellen fraction (for example 6/6, also written 20/20) and a decimal value for each eye.

Please keep its limits in mind. Screen brightness, room lighting, an inaccurate calibration or a wrong distance all shift the result, and a home test cannot detect conditions such as early glaucoma, cataract or retinal disease, which often leave acuity normal until late. Use it as a quick check between professional eye examinations, not as a replacement for them, and never use it to decide on a glasses prescription.

How to use

1. Calibrate first: hold a standard bank or ID card against the screen and resize the box until it matches the card exactly.
2. Set your browser zoom to 100% and turn screen brightness up to a comfortable, fairly high level in a well-lit room without glare.
3. Measure the viewing distance with a tape measure (3 metres by default) and enter it if you use a different distance.
4. Wear your usual distance glasses or contact lenses if you have them, so you check your corrected vision.
5. Cover your left eye with your palm (do not press on it) and answer the direction of each E for the right eye.
6. Do not squint or lean forward; if you are unsure, give your best guess rather than skipping.
7. Repeat with the right eye covered, then compare the two results.

Reading your result

The result is shown as a Snellen fraction such as 6/6 (20/20 in feet) or a decimal such as 1.0. The top number is the test distance, the bottom number is the distance at which a person with typical vision could read the same line. So 6/12 (decimal 0.5) means you need to be at 6 metres to see what a typical eye sees at 12 metres. Many adults with healthy, corrected eyes reach 6/6 or better.

Book an eye examination if either eye scores worse than about 6/12 (0.5), if there is a difference of two lines or more between the eyes, or if your result is worse than the last time you checked. Children who cannot reach the expected level for their age, or who score differently in each eye, should be seen by an eye-care professional, because amblyopia (lazy eye) is treated best early. A good score does not rule out eye disease.

Frequently asked questions

Is 20/20 perfect vision?

20/20 (6/6) means normal sharpness at distance, not perfect eyes. Many people see better than 20/20, and acuity says nothing about peripheral vision, colour vision, focusing or eye health.

Why do I need to calibrate with a bank card?

Screens differ in pixel density, so the same image appears at different sizes on different devices. Matching the card fixes the real size of the letters, which the acuity calculation depends on.

Can I use this test to order glasses?

No. Acuity tells you how well you see, not why. Only a refraction by an optometrist or ophthalmologist can determine a correct prescription.

Can children take the test?

Children who can point in four directions can try it with an adult checking distance and eye covering. It is not a substitute for school or professional vision screening.