Vision Calculator

Visual acuity — the sharpness and clarity of vision — is one of the most fundamental measurements in ophthalmology and optometry. Over the past 160 years, several notation systems have been developed to express visual acuity in standardized ways. The three most widely used systems are Snellen notation, decimal notation, and LogMAR (Logarithm of the Minimum Angle of Resolution). Understanding how these systems relate to one another is valuable for patients, researchers, and healthcare professionals who encounter different conventions depending on geographic region and clinical context.

Snellen Notation

Developed by Dutch ophthalmologist Hermann Snellen in 1862, Snellen notation is the most familiar system in the United States and many English-speaking countries. It is expressed as a fraction, such as 20/20, where the numerator represents the testing distance (20 feet in the US, or 6 meters in countries using the metric system — hence 6/6 is equivalent to 20/20) and the denominator represents the distance at which a person with normal vision could read the same line.

A Snellen measurement of 20/40, for example, means that the person being tested can read at 20 feet what a normally sighted person could read at 40 feet. Conversely, 20/10 means the person can read at 20 feet what normal vision requires being at 10 feet — indicating better than average acuity. Common denominator values used in clinical practice include 20, 25, 30, 40, 50, 70, 100, and 200. In the US, a Snellen acuity of 20/200 or worse in the better eye with best correction is the primary criterion for legal blindness.

Decimal Notation

Decimal notation expresses visual acuity as a simple ratio derived from the Snellen fraction. The calculation is straightforward: divide the numerator by the denominator. Thus, 20/20 becomes 1.0, 20/40 becomes 0.5, and 20/200 becomes 0.1. This system is widely used in Japan, much of continental Europe, and many Asian countries.

The decimal system has the advantage of producing a single intuitive number where 1.0 represents standard normal vision, values above 1.0 indicate better-than-normal acuity, and values below 1.0 indicate reduced acuity. Its simplicity makes it easy to communicate and compare. However, the decimal scale is not linear with respect to the actual change in visual ability — the difference between 0.1 and 0.2 represents a much larger functional change than the difference between 0.9 and 1.0.

LogMAR Notation

LogMAR (Logarithm of the Minimum Angle of Resolution) was introduced to address the non-linearity of the Snellen and decimal systems. It is calculated as the negative base-10 logarithm of the decimal acuity: LogMAR = -log10(decimal). Under this system, 20/20 (decimal 1.0) equals LogMAR 0.0, 20/200 (decimal 0.1) equals LogMAR 1.0, and better-than-normal acuity such as 20/10 (decimal 2.0) equals LogMAR -0.3.

The LogMAR system has several advantages for clinical research. Each line on a LogMAR chart (such as the ETDRS chart) represents an equal logarithmic step, making statistical analysis more meaningful. The scale is continuous and allows for letter-by-letter scoring, which provides finer discrimination between acuity levels. For these reasons, LogMAR is the preferred notation system in most modern vision research and clinical trials.

WHO Classification of Vision Impairment

The World Health Organization (WHO) ICD-11 classification defines categories of distance vision impairment based on presenting visual acuity in the better eye. Normal vision is defined as Snellen 20/20 or better (decimal 1.0 or above, LogMAR 0.00 or below). Mild vision impairment ranges from 20/30 to 20/60 (decimal 0.33 to 0.67). Moderate impairment spans from worse than 20/60 to 20/200. Severe impairment ranges from worse than 20/200 to 20/400. Blindness is defined as visual acuity worse than 20/400 in the better eye.

It is important to note that visual acuity is just one aspect of vision. Other critical factors include visual field (peripheral vision), contrast sensitivity, color vision, and the ability to accommodate (focus at different distances). A comprehensive eye examination evaluates multiple dimensions of visual function beyond acuity alone.

Regional Differences in Practice

The choice of notation system varies significantly by geography and clinical context. In the United States, Snellen notation (20/x) dominates everyday clinical practice. In Japan and much of Asia, decimal notation is the standard — patients typically know their vision as a decimal number (e.g., 0.7 or 1.2). In the United Kingdom, Snellen notation using meters (6/x) is traditional but LogMAR is increasingly adopted. International research publications overwhelmingly favor LogMAR notation due to its statistical properties.

When converting between systems, it is important to understand that all three notations describe the same underlying measurement — they are simply different ways of expressing the same visual acuity. This converter tool allows instant translation between all three systems, making it easy to interpret results regardless of which notation was used in the original measurement.