Add calculated field for power pivot Excel 2010

To Create a Calculated Field in the Power Pivot Window by using the Calculation Area

1. Click Home> View> Calculation Area.
2. Click on an empty cell in the Calculation Area.
3. In the formula bar, at the top of the table, enter a formula in this format <calculated field name>:<formula>, eg rate:=SUM([a])/SUM([b])
4. Press Enter to accept the formula.