Visual Acuity Converter (Snellen 20/20 and 6/6, decimal and logMAR)

Understanding the visual acuity converter — concept and principle

This calculator converts a visual acuity result between Snellen notation in feet (20/20) and metres (6/6), decimal acuity, logMAR and the minimum angle of resolution. It helps compare results from different charts and countries.

Same vision, four scales: with the default inputs the result is logMAR 0.3 (MAR 2 arcmin).

How it is calculated

Decimal acuity = Snellen numerator ÷ denominator (for example 20/40 = 0.5)

Minimum angle of resolution (MAR, arcmin) = 1 ÷ decimal

logMAR = log₁₀(MAR) = −log₁₀(decimal)

6-metre Snellen denominator = 6 ÷ decimal; 20-foot denominator = 20 ÷ decimal

The principle

All notations describe the same quantity: the smallest detail the eye can resolve, measured as an angle. Normal reference vision resolves 1 minute of arc. A Snellen fraction compares the test distance with the distance at which the smallest letters you read would subtend that reference size, so 20/40 means you need to be at 20 feet to read what a reference eye reads at 40 feet. logMAR turns the angle into a log scale where each 0.1 step is one chart line.

Worked example

Default inputs: Snellen 20 ft, 20/40.

Decimal = 20 ÷ 40 = 0.5.
6-m equivalent = 6 ÷ 0.5 → 6/12.
MAR = 1 ÷ 0.5 = 2 arcmin.
logMAR = log₁₀(2) = 0.301 → 0.3.

How to read the result

logMAR 0.0 equals 20/20 (6/6, decimal 1.0); higher logMAR means worse vision, and negative values mean better than reference. 0.3 equals about 20/40, which in many countries is close to the minimum standard for driving, and 1.0 equals 20/200. Printed charts use rounded sizes such as 6/9.5 and 6/7.5.

Common mistakes

Confusing decimal acuity with a percentage of vision.
Treating Snellen steps as equal; logMAR steps are equal, Snellen steps are not.
Comparing results taken with and without glasses.

Keep in mind: This converts notations only; it is not an eye examination or a diagnosis. See an optometrist or ophthalmologist for vision concerns.

Frequently asked questions

Is 20/40 vision bad?

It is below the 20/20 reference; whether it matters depends on the task and local standards.

What is logMAR used for?

Research and clinical trials, because each line is an equal step.

What does 6/6 mean?

It is the metric version of 20/20, tested at 6 metres.

Sources: Bailey IL, Lovie JE, "New design principles for visual acuity letter charts", Am J Optom Physiol Opt 1976;53(11):740–745 · International Council of Ophthalmology, Visual Acuity Measurement Standard, 1984

Checked 9 Nov 2026. NHS Scotland, Visual Impairment Network for Children and Young People (VINCYP), “Conversion Table for LogMAR to Snellen’s Equivalent” (2022): logMAR 1.0 = 6/60, 0.5 = 6/19, 0.3 = 6/12, 0.2 = 6/9.5, 0.1 = 6/7.5, 0.0 = 6/6, −0.1 = 6/5, −0.3 = 6/3, consistent with logMAR = log₁₀(x ÷ 6). Decimal acuity = Snellen fraction and MAR = 1 ÷ decimal are the standard definitions.

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