How can we convert 20/20 feet to 6/6 visual acuity in a Snellen chart?
These are just the different measurement conventions of the U.S. (feet and inches) and the rest of the world using the metric system (meters and cm.). So the answer is “they are the same.”
You do not need to do any conversion since the optotypes (the letters on the eye chart) are already the same shape and size whether called 20/20 or 6/6.
“20/20” means the test is being performed at an equivalent distance of 20 feet (the numerator). This is virtually the same as 6 meters ( 6.096m). The second number (the denominator) is the smallest size line on the chart that can be read. The bigger the denominator, the worse the distance vision.
The 20/40 line is referred to in the metric system as the 6/12 line. So the conversion from one system to the other is the same ratio of numerator to denominator.