Visual Acuity Converter
Optics and Optometry
Snellen, decimal, logMAR and ETDRS letters, in both feet and metres — plus counting fingers and below.
How it is calculated
- logMAR is the base-10 logarithm of the minimum angle of resolution, so logMAR = −log₁₀(decimal acuity), and 20/20 is 0.00 logMAR.
- Decimal acuity is simply the Snellen fraction worked out: 20/40 is 0.5.
- An ETDRS chart has five letters per line and 0.1 logMAR per line, so each letter is worth 0.02 logMAR. With 20/20 anchored at 85 letters, the letter score is 85 − 50 × logMAR.
- One chart line is 0.1 logMAR, which is five ETDRS letters — the unit most trials report change in.
- Counting fingers and hand motion have measured equivalents of 1.98 and 2.28 logMAR, obtained with the Freiburg Visual Acuity Test.
- Light perception and no light perception have no angle of resolution to measure. The values of 2.7 and 3.0 logMAR are agreed conventions for statistics, and the tool labels them as such rather than presenting them as measurements.
References
- Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt 1976;53:740-745.
- Schulze-Bonsel K, Feltgen N, Burau H, Hansen L, Bach M. Visual acuities "hand motion" and "counting fingers" can be quantified with the Freiburg visual acuity test. Invest Ophthalmol Vis Sci 2006;47:1236-1240.
- Moussa G, et al. A novel conversion tool from Snellen fraction to logMAR including CF, HM, LP and NLP. Acta Ophthalmol 2021;99:e963-e967.
About these tools
OcuLearning Clinical Tools is part of an educational platform. These calculators are provided for learning and reference, and are not medical advice, a diagnosis, or a treatment recommendation.