Visual Acuity Converter — Snellen, LogMAR, ETDRS, Decimal

About

Visual acuity (VA) is a quantitative measure of the ability to identify optotypes at a given distance, representing the spatial resolution of the visual system. Several notation systems exist, each with specific clinical applications. Snellen notation (e.g., 20/20 or 6/6) is the most widely recognized standard, representing the testing distance (20 feet or 6 meters) over the letter size identified. LogMAR (Logarithm of the Minimum Angle of Resolution) is the scientific standard, where 0.0 corresponds to 20/20 Snellen and lower values indicate better vision. Each 0.1 LogMAR step corresponds to one line on the ETDRS chart. ETDRS (Early Treatment Diabetic Retinopathy Study) scores range from 0 to 100 letters, with 85 letters equivalent to 20/20 and each letter scored as 0.02 LogMAR. Decimal notation (e.g., 1.0 = 20/20) is commonly used in many countries. The conversion between these systems is precise, using established formulas: LogMAR = -log10(decimal), ETDRS = 85 - 25 × LogMAR, Snellen = 20/10^LogMAR. The LogMAR and ETDRS notations are preferred in clinical research due to their logarithmic nature (equal intervals between lines) making them suitable for statistical analysis and change detection. Snellen remains dominant in clinical practice for its familiarity.

Formula

LogMAR = -log10(decimal). ETDRS = 85 - 25 × LogMAR. Snellen feet = 20 / decimal. Snellen meter = 6 / decimal.

Conversion is based on the established mathematical relationship between notation systems. Enter any ONE of the following: Snellen feet (e.g. 20/40), Snellen metric (e.g. 6/12), LogMAR (e.g. 0.3), ETDRS letter score (e.g. 75), or decimal (e.g. 0.5). The converter will compute all other equivalent values. LogMAR 0.0 = Snellen 20/20 = 6/6 = ETDRS 85 = Decimal 1.0. LogMAR 1.0 = Snellen 20/200 = 6/60 = ETDRS 60 = Decimal 0.1. The VA classification follows WHO/ICD criteria: Normal (20/20 or better), Mild impairment (20/30-20/40), Moderate impairment (20/50-20/80), Severe impairment (20/100-20/200), and Blindness (20/400 or worse).

Score Interpretation

Visual acuity notation conversion is essential in clinical practice and research for several reasons: (1) Different settings use different notations — US clinical practice uses Snellen feet notation, most of the world uses Snellen metric (6m) notation, clinical research uses LogMAR or ETDRS, and European countries often use decimal notation. (2) Clinical trials require standardized notation for endpoint analysis — the vast majority of ophthalmic clinical trials use ETDRS letter scores as the primary endpoint for visual acuity, defining clinically meaningful change as ≥5, ≥10, or ≥15 letter gain/loss (equivalent to 1, 2, or 3 LogMAR lines). (3) Regulatory agencies (FDA, EMA) require ETDRS letter score reporting for ocular drug approvals. (4) The WHO classification of visual impairment uses presenting visual acuity in Snellen notation. The ETDRS chart, designed by Ferris et al. (1982), provides the gold standard for research VA measurement with its logarithmic progression (0.1 LogMAR per line), equal number of letters per line (5), and balanced letter difficulty.

Reference Ranges