Visual Acuity Converter
Convert between Snellen fraction, LogMAR, decimal acuity, and MAR notation for visual acuity measurements.
About this calculator
Visual acuity gets recorded in several different notations depending on the clinical or research context, and this calculator converts between all of them from a single Snellen fraction. Decimal Acuity is simply the Snellen Numerator divided by the Snellen Denominator -- 20/20 becomes 1.0, and 20/40 becomes 0.5, since the patient needs to be twice as close to read what a normal-acuity eye reads at the full distance. MAR (Minimum Angle of Resolution) inverts that ratio -- Denominator over Numerator -- and represents the smallest angular detail, in arcminutes, the eye can resolve; normal 20/20 vision has a MAR of exactly 1 arcminute.
LogMAR takes the base-10 logarithm of that same MAR ratio, which is why 20/20 comes out to a LogMAR of 0.0 and progressively worse vision produces increasingly positive LogMAR values -- this logarithmic, equal-interval scale, introduced by Ian Bailey and Jan Lovie in their 1976 chart-design paper, is why vision researchers and clinical trials favor LogMAR charts over traditional Snellen charts for statistical analysis. The Metric Snellen figure re-expresses the same acuity using the 6-meter testing distance common outside the United States, so a US 20/40 result converts to roughly 6/12 metric. All four notations describe the identical measurement -- only the units and scale differ.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
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Results
Figures current as of 1976. Source: Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt. 1976;53(11):740-745.
How to Use This Calculator
- Enter the Snellen Numerator — the test distance in feet (typically 20 for US charts, 6 for metric).
- Enter the Snellen Denominator — the smallest line the patient reads (e.g., 20 for 20/20, 40 for 20/40).
- The calculator instantly converts to Decimal Acuity, LogMAR, MAR (Minimum Angle of Resolution), and the metric 6/x equivalent.
- LogMAR 0.0 = 20/20 (normal); positive LogMAR values indicate reduced acuity.
- Use these conversions when comparing results across clinical studies, visual acuity charts, or international records.
How the result changes with Snellen Denominator
| Snellen Denominator | Decimal Acuity | LogMAR |
|---|---|---|
| 10 | 2 | -0.3 |
| 15 | 1.33 | -0.13 |
| 30 | 0.67 | 0.18 |
| 50 | 0.4 | 0.4 |
What each input means
- Snellen Numerator
- Test distance in feet (typically 20 for US Snellen, 6 for metric).
- Snellen Denominator
- Smallest line read (e.g., 20 for 20/20, 40 for 20/40). Higher number = worse vision.
How this is calculated
Formula
LogMAR = log₁₀(denominator / numerator)Worked example, using the default values
- Identify Input ParametersSnellen Numerator = 20, Snellen Denominator = 20 = 2 input(s) provided
- Calculate Decimal AcuityDecimal Acuity1 = 1
- Calculate LogMARLogMAR0 = 0
- Calculate MARMAR1 = 1
- Calculate Metric SnellenMetric Snellen6 = 6
Figures and sources
- LogMAR visual acuity notation (1976) — Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt. 1976;53(11):740-745.
Engine last updated . Checked against 4 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does a bigger Snellen Denominator produce a bigger, worse LogMAR value?
LogMAR is the base-10 logarithm of the Snellen Denominator divided by the Numerator, so raising the denominator -- meaning the patient needs a larger, easier letter to read at the same distance a normal eye reads a smaller one -- increases the ratio and therefore increases LogMAR. Since 0.0 LogMAR represents normal 20/20 vision, a larger positive LogMAR value always represents worse acuity.
What does the MAR (Minimum Angle of Resolution) number actually represent?
MAR is the smallest angular gap, measured in arcminutes, that the eye can resolve at the tested acuity -- normal 20/20 vision has a MAR of exactly 1 arcminute. It is calculated as the Snellen Denominator divided by the Numerator, the same ratio LogMAR takes the logarithm of, so MAR and LogMAR always move in the same direction together.
Why would I use the Metric Snellen (6/x) result instead of the US 20/x fraction?
Most countries outside the United States test visual acuity at 6 meters rather than 20 feet, so clinical records and equipment calibrated internationally report acuity as a 6/x fraction. The Metric Snellen output rescales the same measured acuity to that convention, letting you translate a US chart result for an international record or vice versa.
Does entering a Snellen Numerator other than 20 make sense?
Yes -- the Snellen Numerator represents the test distance in feet, and while 20 feet is the standard US testing distance, near-vision cards, pediatric testing, and some international charts use other distances. Entering the actual test distance you used keeps the Decimal Acuity, LogMAR, and MAR conversions accurate to your specific measurement setup.
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