Source data from OLAP databases doesn't allow you to create formulas.
Use a calculated field when you want to use the data from another field in your formula. Use a calculated item when you want your formula to use data from one or more specific items within a field.
To use the data from another field in the formula, click the field in the Fields box, and then click Insert Field. For example, to calculate a 15% commission on each value in the Sales field, you could enter = Sales * 15%.
Add a calculated item to a field
To use the data from an item in the formula, click the item in the Items list, and then click Insert Item (the item must be from the same field as the calculated item).
For example, if a calculated item named Strawberries has a formula of =Oranges * .25 across all months, you can change the formula to =Oranges *.5 for June, July, and August.
To change the formula for several cells, hold down CTRL and click the additional cells.
Note When you add a formula to a PivotChart report or its associated PivotTable report, some chart formatting may be lost.
You can display a list of all the formulas in your report. Click the report, click PivotTable or PivotChart on the PivotTable toolbar, point to Formulas, and then click List Formulas.