Pivot Table: Custom formula in data area
 

I have a pivot table and in the data area that I want to base the
calculations off other rows in the data area. However, when type the = sign I
get a message that says "Cannot change part of a PivotTable report." Is
there any way around this.

To give you an idea I have a field called Allocated Space in the Row area
and in the Column area I have Date. The field I want to calculate is called
"Allocated Space Diffrence" and it's calculated as Allocated for date
selected - Previous month's Allocated. (i.e. 10/1 Allocated Space - 9/1
Allocated Space, 9/1 Allocated Space -8/1 Allocated space, etc)

Pivot Table: Custom formula in data area
 

Try single clicking on your pivot table and then go to
InsertCalculated Field. Then you can use this field in the layout of
your pivot table. Otherwise you can try calculating it just the column
outside (to the right, or below) your pivot table.

Pivot Table: Custom formula in data area
 

Another possibility, which is probably easier, would be to go to your
data which the pivot table is based off of, and create your custom
field with any formula there (i.e. Column A*ColumnB). Then go back to
your pivot table and change the range in the pivot table wizard to
include this new field/column.