Why is the Calculate Geometry tool disabled on a feature class joined with a table?

Answer

The Calculate Geometry tool is occasionally disabled for fields that are joined to a feature class from a table. This is due to the fact that a table does not have geographic features in the map that can be used to calculate geometry from; it does not have geographic properties. The original feature class fields can still have their geometry calculated as long as they are not one of the system-managed fields (ObjectID, Shape_Length, Shape_Area, etc.). If one of the fields from the joined table requires the geometry to be calculated, the feature class should be exported, after the join was successfully established, to a new feature class so that the join is made permanent and those fields are associated with the appropriate geographic features.

The following steps can be used to join a table to a feature class, export it to make the join permanent, and calculate geometry on a field:

1. Join a table to a feature class
   1. In the Table Of Contents, right-click the layer the join is to be applied to, point to Joins and Relates, and click Join.
   2. Click the What do you want to join to this layer? arrow and click Join attributes from a table.
   3. Click the field on which the join is based.
   4. Choose the table to join to the layer. If it is not currently part of the map, click the browse button to search for it on disk.
   5. Click the field in that table on which the join is to be based.
   6. Choose whether to keep all records or only matching records.
   7. Validate the Join.
   8. Click OK.

2. Export the feature class to make the join permanent
   1. Right-click the layer to be exported in the Table Of Contents > Data > Export Data.
   2. Specify the output location and assign a name.
   3. Click OK to export the data.

3. Calculate geometry on one of the joined fields
   1. Start an edit session.

Note:
Calculations can be made without being in an editing session, however in that case, there is no way to undo the results.

2. Right-click the layer created above and click Open Attribute Table.

Note:
Geometric calculations can only be performed on attribute tables.

3. Right-click the field heading to be calculated and click Calculate Geometry.
4. Click the geometric property to calculate.

Note:
Different properties are available depending on the type of layer used.

5. Click to use either the coordinate system of the data source or the coordinate system of the data frame.
6. Click the units of the output calculations.

Note:
If calculating into a text field, adding an abbreviation for units to the calculation can be chosen. For instance, '47.5673 sq m' is an example of the output of area calculated into a text field with the units abbreviation.

7. Optionally, if there are selected records in the table, choose whether to apply the calculations to all records or just the selected ones.
8. Click OK.

Warning:
Field calculation can't be undone when performed outside an edit session.