The conversion part is easy as you say, but the problem lies in which formula to use to determine area. Land parcels can come in all shapes (square, rectangular, circular, trapezoidal, triangular, etc.), with irregular property lines, such as if bordered by a stream or hillside.
More theory in this reply than example unfortunately.
I'm a real estate broker in California, not trying to give you a difficult answer, just that maybe you can look at formulas dealing with geometric shapes as a way to attack this.
Tom Urtis