How to Change Calculation Options from “Manual” to “Automatic” (Excel 2010)
If you have a cell containing a formula, and you have to double click on the cell, and press Enter key to get it to actually calculate, it is probably set to "Manually" calculate. You can change one setting to make the cell automatically calculate. This process shown in Excel 2010, and works the same in Excel 2007 and 2013.