Visual acuity converter
Convert an eye test result between US Snellen (20/x), UK and metric Snellen (6/x), decimal, logMAR and ETDRS letters.
That's about the same as the usual driving minimum (20/40). What a normal eye reads at 20 ft (6 m), this eye reads from about 10 ft (3 m).
What it means
How sharp is that?
For information only: an eye test also depends on the chart, lighting and the examiner. Ask an optometrist about your own eyes.
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That eyesight in every notation
Learn
Three things worth knowing
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Multiply by 0.3
6/x in meters = 20/x in feet × 0.3 (exactly 6/20): 20/40 is 6/12, 20/200 is 6/60.
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Herman Snellen
The Dutch ophthalmologist designed his letter chart in 1862, with letters 5 arcminutes tall at the “normal” distance. The logMAR chart of Bailey and Lovie followed in 1976.
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20/20 is not perfect
20/20 is average. Many young adults see 20/16 or 20/12, and 20/10 is about the human limit.
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Background
About acuity notations
A Snellen fraction puts the test distance over the distance at which a normal eye reads the same line. 20/40 means you need to be at 20 ft to read what a normal eye reads at 40 ft; in meters that is 6/12, and as a decimal 0.5.
Clinics and research use logMAR, the base-10 log of the smallest detail resolved in arcminutes: 0.0 is normal, 0.3 is 20/40, 1.0 is 20/200. Each line on an ETDRS chart is 0.1 logMAR, five letters.