Suitable for eye care professionals.

This app is a very simple tool converting between ETDRS letter count and decimal (Snellen fractions) visual acuity (both ways).

The accuracy of visual acuity measurements is poorer with a Snellen chart compared to ETDRS chart, but in everyday clinical practice it is just Snellen that is most often used. On the other hand there are a lot of clinical trials and assessments where the visual acuity is given in letter count according to ETDRS protocol. Quite a lot of  clinical situations, e.g. dealing with AMD treatment and some other, use ETDRS based letter count results as well. It may desirable to have an easy to use, easy accessible way to “translate” between those two.

Those of you who deal with visual acuity results given in ETDRS letter count and in decimals on regular basis will certainly fluently convert between those two systems on the go. But if you as ophthalmic care professional  only occasionally handle with results of visual tests taken with ETDRS charts, then may benefit from having the opportunity to quickly convert from one to another by just tapping the result on your iPhone or iPad. No settings no hassle, just open the app, input the value and tap ”Calculate” button, to see the result. Simplicity and ease of use make this app useful for your everyday tasks.

There is also a possibility to convert into LogMAR if needed.

The calculation is done using following formulas:

For calculating estimated decimal value of visual acuity from ETDRS letter count:


where x = decimal value of visual acuity, and y = ETDRS letter count

For calculating estimated ETDRS letter count based on decimal visual acuity:

y = 85 + 50 * log(x)

where x = decimal value of visual acuity, and y = ETDRS letter count

From decimal value to LogMAR:

z = -1 * log(x)

where z = logMAR and x = decimal value of visual acuity


Ferris FL 3rd, Bailey I. Standardizing the measurement of visual acuity for clinical research studies: guidelines from The Eye Care Technology Forum. Ophthalmology 1996; 103: 181-182

Ferris FL 3rd, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Opththalmol 1982; 94: 91-96

Gregori NZ, Feuer W, Rosenfeld PJ. Novel method for analyzing Snellen visual acuity measurements. Retina 2010; 30:1046-1050

National Research Council Committee on Vision: Recommended standards for the clinical measurement and specification of visual acuity. Adv Ophthalmol 1980; 41

Snellen H. Probebuchstaben zur Bestimmung der Sehschärfe. Utrecht: P.W. van de Weijer; 1862

Slawomir Janiec, MD




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