2020 Vision Calculator

Health

20/20 vision is shorthand for normal sharpness of sight: you can read at 20 feet what a person with healthy eyes reads at 20 feet. This calculator converts a Snellen test result into the familiar 20/x notation. Enter the distance you were tested at and the distance a normal eye could read the same line from, and it returns your vision ratio. A larger second number means weaker distance vision.

Formula

Snellen Acuity = 20 / (Read Distance / Test Distance) → displayed as 20/X
  • The 20/20 notation compares your sight to a normal eye: the first number is the test distance in feet, the second is the distance a normal eye reads the same line.
  • 20/20 means normal acuity. 20/40 means you must be at 20 feet to read what a normal eye reads at 40 feet — weaker vision.
  • A second number smaller than 20, such as 20/15, means sharper-than-average distance vision.
  • The standard testing distance is 20 feet (about 6 metres), which is why metric charts use 6/6 for the same thing.
  • This is a notation helper, not an eye exam; only an optometrist can measure and diagnose your eyesight.

Example Calculation

Inputs
  • Test Distance (feet): 20
  • Read Distance (feet): 20

Tested at 20 feet, reading the same line a normal eye also reads at 20 feet: acuity = 20/20, which is normal vision. If you can only read at 20 feet a line a normal eye reads at 40 feet, enter readDistance=40 to get 20/40 (below-normal).

Frequently asked questions

What does 20/20 vision mean?
It means you see at 20 feet what a person with normal eyesight sees at 20 feet. It is the benchmark for normal visual acuity, not perfect or above-average sight.
Is 20/20 the best possible vision?
No. Some people have sharper vision, such as 20/15 or 20/10, meaning they can read at 20 feet what a normal eye needs to be closer to read.
What does 20/40 vision mean?
It means your distance vision is weaker than normal: you have to be at 20 feet to read what someone with normal sight reads from 40 feet away.
Why is the testing distance 20 feet?
At about 20 feet (6 metres) the eye is essentially focused at infinity, so the test measures distance vision without the eye having to accommodate.
Does this replace an eye test?
No. It only converts numbers into the 20/x format. A qualified optometrist must perform the actual acuity test and prescribe any correction.