To use formulas efficiently, there are three important considerations that you need to understand: Calculation is the process of computing formulas and then displaying the results as values in the cells that contain the formulas.

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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. When someone tells you that formulas aren’t calculating, it’s probably because the Calculation setting has been changed to Manual, instead of Automatic.

Even more mysterious, the calculations worked fine on some machines, but not others. That can happen if the first workbook that you open in an Excel session was saved as Manual calculation.

In most cases though, the problem is the Automatic Calculation setting.

To see the effects of opening Excel workbooks with different calculation option settings, watch this short Excel tutorial video.

So, I kept tweaking and testing, going through the file one worksheet at a time.

Finally, I found the formulas that seemed to be causing the problem.

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.

Of course, this solution could stop working again, at the next lunar eclipse!