Odd thing with my excel. Sometimes the settings just change for calculation options. I prefer and have set it to Automatic. The random change is to Manuel. No consistent pattern of when. Sometimes every few hours, other times it will be weeks.

When I reboot the laptop it will fix repair the problem and sometime closing and reopening excel will fix it.

I uninstalled the software with Dell support help. It did not really change anything.

does it change only on a per cell/sheet/workbook basis? or does the calculation setting selection actually change from automatic to manual? Are you on Office 365 or Excel 2010/2013?

Do you run macros in any of the sheets that have issues?

I've had the issue of individual cells or worksheets not calculating automatically (or randomly displaying the formula instead of the result) but never the setting physically changing. I'm not on O365 yet, so if you're running that I'm not sure I'll be much help (unless it's macro related).

This is a fairly common problem that often is a result of opening a workbook that has VBA disabling the method or simply has it disabled. In other words, it's a global variable that seems to update when you open up a workbook that is set to manual.

[url=https://social.msdn.microsoft.com/Forums/office/en-US/a98acbee-81c9-4a42-9e19-5f0f7fc1d92b/excel-arbitrarily-changing-calculation-to-manual?forum=exceldev]cHARLES_wILLIAMS MVP on the MSDN forums has a really good synopsis of the issue:

Calculation mode is an Application level property that is initially set from the first non-addin, non-template workbook opened. It is NOT really a Workbook property, although (confusingly) the current Calculation mode is stored in a workbook when it is saved.

So if you start up Excel, set the calculation mode to Automatic, then open a Workbook that has been saved in Manual then it will NOT change back to automatic.

If you then open a second workbook that was saved in Automatic mode Excel will still be in Manual mode (the second workbook does not change anything).

If you then save the second workbook it will be saved in Manual mode. So if people are not careful Manual Calc mode spreads like a virus.

According to this user, you can put VBA in the ones you want to update automatically to force it:

<LINK_TEXT text="https://answers.microsoft.com/en-us/mso ... feaaf2a71a">https://answers.microsoft.com/en-us/msoffice/forum/msoffice_excel-mso_other-mso_archive/excel-sometimes-sneakily-changes-automatic/d9d29b5e-e09d-4fd6-8001-2efeaaf2a71a</LINK_TEXT>

References:

<LINK_TEXT text="https://answers.microsoft.com/en-us/mso ... 5c897e6e50">https://answers.microsoft.com/en-us/msoffice/forum/msoffice_excel-mso_other-mso_o365b/excel-changing-from-automatic-to-manual/a8363a90-2cfb-4870-935b-625c897e6e50</LINK_TEXT>

<LINK_TEXT text="https://answers.microsoft.com/en-us/off ... 962b2?db=5">https://answers.microsoft.com/en-us/office/forum/office_2013_release-excel/workbook-calculation-default-keeps-changing/b6e56980-826b-4c5f-97b8-a94d192962b2?db=5</LINK_TEXT>

<LINK_TEXT text="https://answers.microsoft.com/en-us/off ... 911eea812e">https://answers.microsoft.com/en-us/office/forum/office_2007-excel/the-calculation-option-keeps-switching-from-auto/2ed29346-b946-4aaf-9c19-83911eea812e</LINK_TEXT>

Macros could be the problem. I have some macros that are many years old and have been edited to work though a few versions of excel. Old would probably explain the sometimes issue.