Calculators · Ophthalmology · Conversion

Visual acuity conversion (Snellen, decimal, logMAR)

Also called Snellen to logMAR, 6/6 to 20/20, decimal acuity, MAR

Converts visual acuity between metric (6/x) and imperial (20/x) Snellen, decimal and logMAR.

I am entering

e.g. 6 for 6/18, 20 for 20/40, 3 for 3/60.

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Formula and scoring

Decimal = numerator ÷ denominator. MAR = 1 ÷ decimal (arc-min). logMAR = log10(MAR) = −log10(decimal).

Exam and ward pearls

  • Average acuities only in logMAR — averaging Snellen fractions or decimals is mathematically wrong (Holladay 1997).
  • Converting 3/60 gives decimal 0.05, logMAR 1.3 — the same as 6/120.

When not to rely on it

  • Counting fingers, hand movements and light perception have no exact logMAR equivalent; do not force them into this conversion.

Source

  1. Holladay JT. Proper method for calculating average visual acuity. J Refract Surg. 1997;13(4):388-91. doi:10.3928/1081-597X-19970701-16 · PMID 9268940

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

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