Excel 2010 does not re-calculate formulas automatically

Normally Excel will automatically recalculate formula when data in cells it is referring to changes . Users expect this behaviour and get confused if this doesn’t happen.

On some complex spreadsheets automatic recalculation may be disabled to improve performance. When automatic recalculation is disabled, formulas will not be recalculated if data in referenced cells changes.

Let’s say you have values:
A1 2
B1 3
C1 SUM(A1+B1), which obviously display 5.
If you now change B1 value to 8, C1 will still display 5, which is obviously incorrect.
Automatic Calculation in Excel 2010
Other example:
Column A and B hold various numbers
C1SUM(A1+B1), which in this case is 5
If you now drag cell C1 down, it will put correct SUM formula in each cell, but value in each cell will stay as 5.
Automatic Calculation in Excel 2010

To re-calculate formulas in such spreadsheet:

F9
– recalculates only cells in the workbook that need to be recalculated
SHIFT+ F9 – recalculates only the active sheet
CTL+ALT+F9 – does a full recalculation, regardless of whether cells need to be recalculated

To re-enable automatic calculation:

File > Options > Formulas – set Workbook Calculation to Automatic
Automatic Calculation in Excel 2010