How to enable disabled Area calculation in ArcGIS | Area Calculation | Disabled Area Calculation.
The Calculated Geometry tool allows you to access the geometry of features on a layer. The tool can calculate coordinate values, lengths and areas depending on the geometry of the input layer. You can calculate the area, length, or perimeter of features only by projecting the coordinate system used. Remember that different projections have different spatial properties and distortions. If the coordinate system of the data source and data frame are not the same, you might get a different result if you calculate the geometry using the coordinate system of the data frame than if you calculate it using the coordinate system of the data source. It is recommended to use equal-area projection when calculating areas.
Steps to Follow :-
Start an edit session. ...
Right-click the layer and click Open Attribute Table. ...
Right-click the field heading for which you want to make a calculation and click Calculate Geometry. ...
Click the geometric property you want to calculate.