In data processing, data are usually obtained through coordinate conversion. The cadastral parcel is one of the most important objects, and the area of a parcel is the key attribute with legal authorization. However, the parcel area based on the conversion data is different from the original area. In this paper, the authors have developed a relational equation of area computation with coordinate conversion parameters, which is a simple and convenient method for area computation. Theoretical analysis and case studies show the correctness of the equation of area calculation with coordinate conversion parameters. It is concluded that the area conversion parameters in area adjustment are easier to be detected and hence the scheme of the coordinate conversion can be selected correctly.