A ring with a radius of 10 yards . . .
-- has a diameter of 2·r = 20 yards,
-- has a circumference of 2·π·r = approx. 62.83 yards
-- has an area of π·r² = approx. 314.2 square yards.
-- The total amount of grass that grows inside the ring depends on
the length of the growing season in that location, and also on the
length of time that horses occupy the ring.