If you are using PivotTables against an OLAP data source you may find it useful to be able to create your own custom, calculated measures and sets. These calculations are client-side meaning they exist inside Excel and you don't have to change the server-side OLAP cube.
Excel does not enable you to do this using the user interface, however, you can do this via the API.
Allan Folting (our PivotTable expert) has published a post showing how to do this using VBA over in the team's blog.
Click here to read his post.