Excel has 3 basic calculation modes: Automatic, Manual and Automatic except Tables.
But an Excel session can currently only have one Calculation mode (unless you are using FastExcel which allows open workbooks and their worksheets to have different Calculation Modes).
Excel chooses the session calculation mode from the first workbook opened (the calculation mode is stored in every saved workbook).
Viral Manual Calculation Mode in previous Excel Versions
If the first (non-Addin) workbook you open was saved in Manual mode then the Excel session stays in Manual mode even when you open another workbook that was saved in Automatic mode (Of course you can change Excel’s calculation mode using the Ribbon or VBA).
In Excel versions before Office 365 or Excel 2021:
- Start Excel
- Open a Manual Workbook
- Open an Automatic workbook
- Save the Automatic Workbook
- Close Excel and then re-open the saved Automatic Workbook
- Excel is now in Manual Mode, the saved workbook has been changed to Manual!
If your users are in the habit of having multiple workbooks open at the same time and one of these workbooks is in Manual mode then Manual mode will soon spread virally to the other workbooks.
Excel 365 and Excel 2021
The change in Excel 365 and Excel 2021 is not to change the calculation mode of a workbook when saving it unless the user (or VBA) has changed Excel’s calculation mode before the save.
- Start Excel 365 or 2021
- Open a Manual Workbook
- Open an Automatic workbook
- Save the Automatic Workbook
- Close Excel and then re-open the saved Automatic Workbook
- Excel is now in Automatic Mode, the saved workbook has NOT been changed to Manual!
Excel and UDF Performance Stuff 