How to use the online eye chart
- Calibrate: hold a bank card or ID card upright against the screen and drag the slider until the box matches the card's short edge (53.98 mm), then save. Without this, letter sizes can be off by 20% or more.
- Set the distance you will stand from the screen. 3 metres (10 feet) works well for a laptop or TV; the letters are scaled so the chart is correct at that distance. If your room is small, use mirror mode: put the screen next to a mirror and read the reflection, counting the distance to the mirror and back.
- Test each eye: cover your left eye, read the line aloud, and tap I can read this line (or ask a helper to tap). The next line is smaller. When you cannot read a line, tap I can't read it; your score is the smallest line you read. Then repeat for the left eye.
- Switch to Full chart to see every line at once, for example on a TV for the whole family.
What do 20/20 and 6/6 mean?
Snellen notation compares your vision with a standard eye. 20/20 (6/6 in metres) means you can read at 20 feet what a person with normal vision reads at 20 feet. 20/40 (6/12) means you need to be at 20 feet to read what a normal eye reads at 40 feet. Smaller second numbers are better: 20/15 is sharper than average. In decimal notation 20/20 is 1.0, and in logMAR, used in research, it is 0.0.
A 20/20 letter is exactly 5 minutes of arc tall (1/12 of a degree) at the viewing distance: 8.7 mm at 6 metres, or 4.4 mm at 3 metres. This chart calculates each line from that rule, your distance and your screen calibration, and uses the ten Sloan letters (C, D, H, K, N, O, R, S, V, Z) found on clinical charts, in random order so you cannot memorise them.
Many countries require about 20/40 (6/12) with both eyes, with glasses if needed, to drive. This check cannot test near vision, eye health or field of vision, and screen glare or a shaky helper can affect it, so see an optometrist every one to two years.
How far should I stand from the eye chart?
Any distance from about 1.5 to 6 metres works, as long as you enter it. 3 metres (10 feet) is a good choice for most rooms.
Is an online eye test accurate?
With a calibrated screen and the correct distance, it gives a reasonable estimate of distance acuity, but it is not a substitute for a professional eye exam.
What is normal vision?
20/20 (6/6) is considered normal. Many young adults see 20/15 or better.
Should I wear glasses for the test?
Wear them to check how well your current glasses work, or take them off to see your uncorrected vision.