The word acre is defined as meaning open field. The acre was approximately the amount of land estimated tillable by one man plowing behind an ox in one day. This explains one definition as the area of a rectangle with sides of length one chain (66 feet) and one furlong (660 feet). In reality, a long narrow strip of land is more efficient to plow than a square plot, since the plow does not have to be turned so often. The word furlong (most commonly used in horse racing) itself comes from the fact that it is one furrow long.
Other area calculators and volume calculators can be found here.