What happens when good Excel formulas go bad? A workbook of mine that worked fine for several years, when using Excel 2003, suddenly refused to update all the formulas, after a switch to Excel 2010.

Even more mysterious, the calculations worked fine on some machines, but not others. So, what was the problem?

Hint: It wasn't one of the obvious solutions.

Check for Automatic Calculation

When someone tells you that formulas aren't calculating, it's probably because the Calculation setting has been changed to Manual, instead of Automatic. That can happen if the first workbook that you open in an Excel session was saved as Manual calculation. That setting affects all the other workbooks the you open during that session.

In this case, the workbook calculation was set to Automatic on all the machines – that's the first thing we checked. To check the setting, click the Formulas tab on the

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