Add an area or perimeter field to a shapefile or feature class in ArcGIS Desktop

Typically, polygon feature classes in ArcGIS already have a length and area field defined. However, if you do not see this information reported in the attribute table, you can add a new field to an existing polygon feature class or shapefile.

To create a new field containing values for each feature's area, perimeter, or x and y coordinate of centroid:

1. Verify that the data layer has a coordinate system. In ArcMap, right-click the data layer and select Layer Properties, and then choose the Source tab. If the coordinate system is undefined, you will need to project the data before continuing.
2. To open the layer's attribute table, right-click the data layer, and then select Open Attribute Table.
3. In the attribute table, click Options and select Add field. ArcGIS allows you to add a new field only if you are not in an editing session.
4. Name the new field, specifying the field type as either float or double, according to your needs. A new field is added with null values for each feature.
5. Begin an editing session for this feature class. Using the Editor toolbar, select Start Editing. If you do not see the Editor toolbar, select View from the main menu of ArcMap, click Toolbars, and then select Editor.
6. In the attribute table, right-click the new field heading (the name of the field at the top of the table), and select Calculate Geometry.
7. In the "Calculate Geometry" window, select the type of calculation: Area, Perimeter, x coordinate of centroid, or y coordinate of centroid.
8. Choose the coordinate system and units.
9. Click OK. The new field will be populated with the calculation for each feature.
10. To preserve values in the new field, save the editing session.

You can also calculate area values for a polygon feature class by using the Calculate Areas tool in ArcToolbox. Under "Spatial Statistics Tools", select Utilities and then Calculate Areas. This tool will create a new polygon feature class with an "Area" field and feature values. Make sure to save the new feature class within a geodatabase.

For more, see ArcGIS Desktop Help. Search using the term calculate areas. To display details, select the first listed item, Calculate Areas (Spatial Statistics).