Visual acuity converter: 20/20, 6/6, decimal and logMAR

How to use it

A US school form might say 20/40, a British clinic letter 6/12, an optician's card in Japan or much of Europe 0.5, and a research paper 0.3 logMAR. All four describe the same line on a chart.

Choose the format you have and type the number. For a Snellen score, enter only the second figure: 40 for 20/40, 12 for 6/12. Decimal scores go in as written (0.5, 1.0, 1.2). LogMAR values can be negative, because anything below zero is sharper than 20/20.

You get all four formats, the minimum angle of resolution (MAR) in minutes of arc, the nearest standard chart line and a short reference note.

Convert one eye at a time. OD or R is the right eye, OS or L the left; "cc" means with glasses or contacts and "sc" without.

How the numbers work

Every format comes from one measurement: the smallest detail you can make out, expressed as an angle. On the 20/20 line each letter is 5 minutes of arc tall and each stroke 1 minute (a minute of arc is a sixtieth of a degree). A score of 20/40 means you need to be at 20 feet to read what the reference eye reads at 40, so detail must be twice as large.

For a score of 20/x:

• MAR = x ÷ 20
• decimal = 20 ÷ x
• logMAR = log10(MAR)
• metric score = 6/(0.3 × x), treating 6 meters as 20 feet

Other inputs become 20/x first: from 6/y, x = 20 × y ÷ 6; from a decimal d, x = 20 ÷ d; from logMAR L, x = 20 × 10 to the power L. Results are rounded to two decimal places, the metric figure to one.

The tool opens on 20/40. The MAR is 40 ÷ 20 = 2, the decimal score 20 ÷ 40 = 0.5, the logMAR log10(2) = 0.30 (shown as 0.3), and the metric score 6/12. The nearest line is 20/40 itself, and the note points to the common US driving standard.

Why charts step in 0.1 logMAR

Charts built on the Bailey-Lovie and ETDRS designs make each line about 1.26 times larger than the one below, a step of exactly 0.1 logMAR. Even steps mean that going from 20/20 to 20/25 is the same size of change as going from 20/160 to 20/200. With five letters on every line, each letter counts as 0.02 logMAR, so research studies prefer them.

A chart only has whole lines, so the tool also shows the nearest one. Enter 6/9, a familiar line on older charts, and the exact conversion is 20/30, decimal 0.67, logMAR 0.18. The closest logMAR line is 20/32 (logMAR 0.2), so that appears as the nearest line. The tool's list runs in 0.1 steps from 20/10 to 20/200, then jumps to 20/400.

Some US figures are rounded themselves (20/32 is really about 20/31.7), so printed metric charts often label those lines 6/9.5 and 6/19 rather than 6/9.6 and 6/18.9. They are the same lines.

What the notes mean

• Better than 20/20: sharper than the reference line.
• 20/20: the line usually called normal vision.
• Up to 20/40: many US states use 20/40 in the better eye for a regular driving license. Rules vary by state and country, and many also check the field of vision.
• Between 20/40 and 20/200: below the common driving standard; an eye exam can show whether glasses or treatment would help.
• 20/200 or worse: in the US, legal blindness means 20/200 or less in the better eye with the best possible correction, or a field of vision 20 degrees wide or less. A score without glasses doesn't count.

What this tool can't tell you

The conversion is exact; the measurement behind it never is. Lighting, distance and guessing can each move a home result by a line or more, so the printable eye chart is a screening aid, not a diagnosis.

Acuity measures only how sharp high-contrast letters look straight ahead. It says nothing about side vision, color or the health of the retina and optic nerve; early glaucoma can take away side vision while the chart still reads 20/20.

See an eye doctor if a score has dropped since your last test, if one eye is clearly worse, if you can't reach 20/40 with your glasses, or if a child's screening result is borderline (a weak eye treated early usually does much better). The American Academy of Ophthalmology recommends a baseline eye exam at 40 for adults without symptoms; your first eye exam explains what happens.

Some changes can't wait:

• New flashes of light, a sudden shower of floaters, or a shadow or curtain over part of your vision need same-day eye care.
• Sudden loss of vision in one eye, even if it returns within minutes, can be a warning sign of stroke. That, or severe eye pain with blurred vision, halos, headache or nausea, means going to an emergency department or calling your local emergency number (such as 911, 999, 112, or 119 in Japan).

Eye emergencies and first aid covers what to do on the way.

Questions people ask

Is 20/20 perfect vision?

No. It is the line a typical healthy eye reads. Many people see better, and someone with 20/20 can still have glaucoma or a color vision difference.

Can I work out my glasses prescription from my score?

Not reliably. Two people with the same prescription can read different lines, and the same score can come from nearsightedness, astigmatism or an eye condition glasses won't fix. The glasses prescription reader explains the numbers on a prescription instead.

What does 20/30-2 mean?

You read the 20/30 line but missed two letters on it. A "+2" means you read the line plus two letters on the next smaller one. The converter works with whole lines, so enter the line itself.

Why is my logMAR score negative?

You read smaller than the 20/20 line. Each 0.1 below zero is one line better, so -0.1 is 20/16 and -0.2 is 20/12.5. Higher logMAR numbers mean weaker vision, the opposite of the decimal scale.

Sources

• Bailey-Lovie (1976) and ETDRS logMAR chart design
• ISO 8596, the standard for acuity testing with the Landolt ring
• The US Social Security Administration definition of statutory (legal) blindness
• American Academy of Ophthalmology patient pages (aao.org/eye-health)