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Double-click the Excel document that contains your pivot table. If you haven't yet made the pivot table, open a new Excel document and create a pivot table before continuing. Click the Pivot Table on your worksheet to select and edit it. This tab is in the middle of the toolbar ribbon at the top of the Excel window. It will open your pivot table tools on the toolbar ribbon. On different versions, this tab may be named Analyze, or Options under the "Pivot Table Tools" heading. This button looks like an "fx" sign on a table icon on the far-right end of the toolbar. It will open a drop-down menu. It will open a new window where you can add a new, custom column to your Pivot Table. Click the Name field, and type in the name you want to use for your new column. This name will appear at the top of the column. Click the Formula field below Name, and type the formula you want to use for calculating you new column's data values.  Make sure you type the formula on the right side of the "=" sign. Optionally, you can also select an existing column, and add it to your formula as a value. Select the field you want to add in the Fields section here, and click Insert Field to add it to your formula. Doing so will add the column to the right side of your pivot table.
Open the Excel document you want to edit. Select the pivot table you want to edit. Click the Pivot Table Analyze tab. Click the Fields, Items, & Sets button on the toolbar ribbon. Click Calculated Field on the drop-down menu. Enter a name for your column in the "Name" field . Enter a formula for your new column in the "Formula" field. Click OK.