Optometry tools
Snellen Acuity Estimator
Determine approximate visual acuity from the smallest optotype correctly read at a given test distance. The defaults correspond to the classic 20/20 (6/6) line using Sloan letters (~8.73 mm height at 20 ft).
Standard Snellen charts use 20 ft (6 m).
Height of the smallest letters correctly identified.
Enter the real distance between patient and chart (for near charts or limited room length). Adjusted Snellen fraction accounts for this distance.
Baseline Snellen (test distance)
20/2399594
Equivalent decimal: 0.00
Adjusted distance Snellen
20/2399594
Decimal: 0.00
Acuity worse than 20/70. Seek ophthalmic evaluation and consider low-vision assessment.
How to Use This Calculator
Measure optotype size
Determine the physical height (mm) of the smallest line correctly read (e.g., on an ETDRS or Snellen chart).
Record test distance
Enter the nominal chart distance (20 ft/6 m) and the actual measured distance if different (e.g., due to mirror setups).
Interpret the acuity
Review Snellen and decimal outputs, and use the clinical interpretation to decide on further testing or referral.
Formula
Snellen denominator ≈ (Optotype height / Test distance)⁻¹ × 3438 (arcmin)
Decimal acuity = Test distance (ft) ÷ Snellen denominator
Adjusted Snellen = (Measured distance ÷ Standard 20 ft) × Snellen denominator
Optotype height corresponds to five minutes of arc for the gap of Sloan letters; the calculator uses small-angle approximations.
Full Description
Snellen visual acuity compares the testing distance with the distance at which a person with standard vision would resolve the same optotype. Modern charts (ETDRS) use logarithmic spacing and Sloan letters, but many clinical and screening environments still reference 20/20 notation. This tool bridges physical optotype dimensions and Snellen fractions, useful for custom charts or near-vision adaptations.
For comprehensive exams, combine acuity measurements with refraction, contrast sensitivity, and ocular health assessments. Always document whether vision was tested with correction, and note any pinhole or refraction performed during evaluation.
Frequently Asked Questions
How do I convert to LogMAR?
LogMAR = −log₁₀(decimal acuity). For example, 20/40 (decimal 0.5) equals LogMAR 0.30.
Does this work for near-vision cards?
Yes. Enter the near working distance (e.g., 16 inches = 1.33 ft) and optotype height. Resulting Snellen shows equivalent distance acuity.
Why adjust for actual distance?
Short rooms often use mirrors or shorter distances; adjusting prevents overestimating acuity when the chart is closer than standard.
What if the patient misses some letters?
Snellen scoring typically records the line read and letters missed (e.g., 20/25−2). Use clinical judgment to interpret borderline performance.