Adding a cumulative sum column to a pivot table
Have a look at this topic on the Contextures site:
Excel Pivot Table Tutorial -- Running Totals
Can't reply to your comment in Doug's post as I don't have enough points, but just drag the field to the data area alongside the running total column. You can have the same field appear more than once on a pivot table.
Running Total is a built in function for Pivot Tables.
Under Value Filed Settings you can find the settings you need.
On the Summarize Values By tab, Select SUM.
Then on Show Values As tab select Running Total In, And Select the column header you wish to Group By.
