20/20, 6/6, 1.0 and 0.00 are the same eyesight

What the fraction actually says

In 20/40, the top number is the distance you were tested at and the bottom is the distance at which a typical eye resolves the same letter. So 20/40 means you read at twenty feet what an unimpaired eye reads at forty. A larger bottom number means less resolving power.

The metric version works identically at a six-metre test distance, which is why 6/6 and 20/20 mean exactly the same thing, and 6/12 equals 20/40.

Minimum angle of resolution

Underneath all four notations is a single physical quantity: the smallest angle, in minutes of arc, at which you can tell two things apart. A typical eye resolves about one arcminute, which is where the 20/20 reference comes from. 20/40 is two arcminutes, 20/200 is ten.

Decimal notation is simply the reciprocal of that: 1.0 for one arcminute, 0.5 for two. It is the standard in much of continental Europe and Japan.

Why research uses logMAR

Snellen lines are unevenly spaced. The gap between 20/20 and 20/25 is not the same size as the gap between 20/100 and 20/200, so you cannot average two Snellen results or subtract one from another and get anything meaningful. This matters enormously in a clinical trial, where the question is usually how much vision changed.

LogMAR takes the base-ten logarithm of the minimum angle of resolution, which turns the scale linear. 20/20 becomes 0.00, 20/200 becomes 1.00, and better-than-reference acuities go negative: 20/16 is about −0.10. On a properly constructed chart each line is exactly 0.1 logMAR, so the numbers can be averaged and analysed. That is why every serious ophthalmic study reports logMAR while your optician quotes a fraction.

The impairment scale, and the thing everyone gets wrong

The WHO categories for distance vision impairment are defined in metric Snellen: mild is worse than 6/12 up to 6/18, moderate is worse than 6/18 up to 6/60, severe is worse than 6/60 up to 3/60, and blindness is worse than 3/60. These all refer to presenting vision in the better eye.

US legal blindness is a completely separate threshold serving a completely different purpose: 20/200 or worse in the better eye with best correction, or a visual field of twenty degrees or less. It is an administrative definition for benefits eligibility, not a clinical category.

The two get conflated constantly, and the correction clause is the part that vanishes. Someone whose uncorrected vision is 20/600 and who sees 20/20 in glasses is not legally blind, has never been legally blind, and is nowhere near it. If you have read that a −5.00 prescription makes you legally blind, that is where the claim comes from and it is simply wrong.

Driving standards

Most US states and many European authorities set 20/40, or 6/12, in the better eye, measured with whatever correction you normally wear. Some add a visual field requirement, commercial licences are stricter, and the details vary enough that the only sensible move is to check your own licensing authority.

What acuity does not measure

A single number describing high-contrast black letters on white under good lighting leaves out most of vision. Contrast sensitivity governs how you see in fog, at dusk and against low-contrast backgrounds, and can be poor in an eye with excellent acuity. Visual field, colour discrimination, depth perception, glare recovery and how quickly the eye adapts to darkness are all separate.

This is worth knowing because people sometimes conclude from a good acuity figure that their eyes have been checked. They have not.

This tool converts between notations. It does not measure your vision, does not diagnose anything, and cannot substitute for an eye examination. Thresholds quoted for driving and legal definitions vary by jurisdiction and change; verify with the relevant authority.

Related: read your prescription, or convert between contacts and glasses powers.

Questions people ask

Is 20/20 perfect vision?

No, it is the reference standard, not a ceiling. It means you resolve at twenty feet what a typical eye resolves at twenty feet. Plenty of healthy young eyes reach 20/15 or 20/12, and acuity is only one aspect of vision — it says nothing about contrast sensitivity, colour, peripheral field or depth.

What is 20/20 in metric?

6/6. The notations are identical in meaning; one is measured in feet and the other in metres at a six-metre test distance. 20/40 is 6/12, 20/60 is 6/18, and 20/200 is 6/60.

Why do researchers use logMAR instead of Snellen fractions?

Because Snellen lines are unevenly spaced, so you cannot meaningfully average them or compute a difference. LogMAR is the base-ten logarithm of the minimum angle of resolution, which makes the scale linear: each 0.1 step is one line on a properly constructed chart, and the numbers can be averaged, subtracted and analysed statistically.

Does 20/200 mean legally blind?

Only under specific conditions that are almost always left out. The US definition is 20/200 or worse in the better eye with best correction in place, or a visual field of twenty degrees or less. It refers to corrected vision. Someone whose uncorrected vision is 20/400 but who sees 20/20 in glasses is not legally blind, and this is probably the most frequently misquoted figure in all of eye care.

What acuity do I need to drive?

Most US states and many European authorities set 20/40 in the better eye, and it applies to your vision with glasses or contacts on. Requirements vary by jurisdiction and by licence class, and some add a visual field requirement, so check your local authority rather than relying on a general figure.

Can I test my acuity on a screen?

Not reliably, and this tool deliberately does not try. Screen-based testing depends on exact viewing distance, display pixel density, brightness, contrast and calibration, and errors in any of those change the result by whole lines. More importantly, an acuity number is not an eye examination: the examination is where glaucoma, retinal changes and other conditions get found, usually in people with no symptoms.