How this instrument works
Visual acuity — how well someone can resolve fine detail at a standard distance — gets recorded in more than one notation, and the ones below all describe the same underlying measurement. Snellen notation, the familiar fraction from a standard eye chart, compares the distance at which a person can read a line (the numerator) to the distance at which a person with typical vision could read that same line (the denominator) — 20/40 means reading at 20 feet what a standard eye reads at 40 feet.
Decimal acuity is simply that fraction's numeric value — the Snellen numerator divided by the denominator — and is the everyday notation used in much of Europe. LogMAR (log of the minimum angle of resolution) is the base-10 logarithm of the Snellen fraction's reciprocal (denominator ÷ numerator); it's the notation clinical researchers prefer because, unlike a Snellen fraction, its steps are evenly spaced across the acuity range, which makes it better suited to statistical averaging and analysis (Holladay, 1997).
On the LogMAR scale, 0 is normal (20/20) acuity, and lower — including negative — values mean better-than-average acuity; higher, positive values mean worse acuity. 20/200, which converts to a LogMAR of 1.0, is a commonly cited reference point for 'legal blindness' in the better eye with best correction — but the precise legal or regulatory definition of blindness varies by country and by the specific benefit or program in question, and this calculator only performs the notation conversion; it isn't a legal or clinical determination.
- Enter the Snellen fraction numerator (usually 20, or 6 if using metres).
- Enter the Snellen fraction denominator — the line read on the eye chart (e.g. 20, 40, 200).
- Read the equivalent decimal acuity and LogMAR values.
Worked example — 20/20, 20/40, and 20/200
20/20 — the numerator equal to the denominator — converts to a decimal acuity of exactly 1.0 and a LogMAR of exactly 0, the reference point both other notations are built around.
20/40 — needing to stand at 20 feet to read what a standard eye reads at 40 feet — converts to a decimal acuity of 0.5 and a LogMAR of log₁₀(2) ≈ 0.301.
20/200 converts to a decimal acuity of 0.1 and a LogMAR of exactly 1.0. This is the fraction commonly referenced as a threshold for legal blindness (in the better eye, with best correction) in some legal and regulatory contexts — though the exact definition and its uses vary by jurisdiction.
Questions
What's the difference between Snellen, decimal, and LogMAR acuity?
They're three notations for the same measurement. Snellen is the familiar distance-based fraction (e.g. 20/40) from a standard eye chart. Decimal acuity is that fraction's numeric value (20/40 = 0.5). LogMAR is the base-10 logarithm of the fraction's reciprocal, used mainly in clinical research because its scale steps are evenly spaced, unlike Snellen's.
Why do researchers use LogMAR instead of Snellen fractions?
Snellen fractions don't step evenly — the visual difference between 20/20 and 20/25 isn't the same size as between 20/100 and 20/125, even though both pairs look like similar-sized jumps. Holladay (1997) describes how LogMAR's log scale gives uniform step sizes across the acuity range, which makes it more suitable for statistical averaging and for comparing results across studies.
What does a LogMAR of 0 mean? Can LogMAR be negative?
0 LogMAR corresponds to exactly 20/20 (decimal 1.0), the reference point the scale is built around. Because LogMAR is a logarithm, acuity better than 20/20 — such as 20/16 — produces a small negative LogMAR value (log₁₀(16/20) ≈ -0.1); on this scale, lower and negative numbers mean better acuity, the opposite of how bigger Snellen denominators mean worse acuity.
Is 20/200 the official legal definition of blindness?
It's a widely cited reference point — for example, it has historically been used in U.S. tax and disability-benefit eligibility criteria as 'central visual acuity of 20/200 or less in the better eye with best correction' — but exact legal and regulatory definitions of blindness vary by country and by the specific program involved. This calculator performs the notation conversion only; it doesn't make a legal or clinical determination.
Can this calculator convert acuity recorded as 'counting fingers' or 'hand motion'?
No. It only converts a standard Snellen fraction (numerator over denominator) to decimal and LogMAR. Very low vision is sometimes recorded with non-fraction clinical notations like counting fingers (CF), hand motion (HM), or light perception (LP), which this calculator doesn't handle.
Does this calculator assess or diagnose my vision?
No. It converts a Snellen fraction you already have — from an eye chart, a prescription, or a clinical record — into decimal and LogMAR notation. It doesn't measure vision itself and isn't a substitute for an eye exam.
References
- Holladay JT. Proper method for calculating average visual acuity. J Refract Surg. 1997;13(4):388-391.
- Low Vision and Legal Blindness: Terms and Descriptions, American Foundation for the Blind
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.