Visual Acuity Converter: Snellen, Decimal, ETDRS & logMAR

A clinic records 20/40, a study reports logMAR, a colleague writes 0.5 decimal — three ways of saying the same vision. Comparing a chart note with a paper, or recording acuity in logMAR for research, means converting between notations, and doing it in your head is error-prone. This free converter does it instantly, in clinic or at the desk, and works fully offline once installed on your phone or tablet.

• Reading records across notations: older notes in decimal, charts in Snellen, research data in logMAR.
• Research and trials: logMAR and ETDRS letter scores are the standard for analysis, so clinic values have to be converted in.
• Comparing charts or devices that report acuity differently.
• Teaching and exams:trainees are routinely asked to move between Snellen, decimal and logMAR.

Snellen is the familiar fraction — 20/20 in feet, or 6/6 in metres. The top number is the testing distance; the bottom is the distance at which a normal eye reads that line. Decimal expresses the same thing as a single number (20/20 = 1.0). logMAR is the logarithm of the minimum angle of resolution: 20/20 is 0.00, lower is better, and each 0.1 step is one chart line — which is exactly why research prefers it, since it can be averaged and analysed properly. ETDRS counts letters read on the standard chart: 85 letters equals 20/20, and each line is 5 letters, so it maps directly onto logMAR (letters = 85 − 50 × logMAR).

• Open the converter. Tap the button above. Add it to your home screen for full-screen, offline use in clinic.
• Enter a value in any notation.
• Type the acuity you have — Snellen, decimal or logMAR — and the converter fills in the others instantly.
• Copy what you need. Take the notation your record or study requires.

20/20 (6/6) equals 0.00 logMAR and 1.0 decimal — normal reference acuity.

Take the logarithm (base 10) of the minimum angle of resolution, which is the Snellen denominator divided by the numerator. For 20/40 that is log10(2) = 0.30.

logMAR is a continuous, linear scale where each 0.1 is one chart line, so values can be averaged and analysed statistically — something Snellen fractions cannot do reliably.

It expresses the same vision as a single number rather than a fraction: 20/20 is 1.0, 20/40 is 0.5, and so on.

It is the metric equivalent of 20/20 — normal acuity, measured at 6 metres instead of 20 feet.

On a standard 4-metre ETDRS chart, 20/20 (logMAR 0.00) corresponds to 85 letters. Each chart line is 5 letters, so 20/40 is 70 letters and 20/200 is 35 letters.

The logMAR scale comes from the chart design principles described by Bailey and Lovie, which gave each line an equal logarithmic step and made acuity suitable for statistical analysis.