Option should be selected to instruct Excel to manually calculate all open workbooks is F9.
All open workbooks' formulas will be calculated as follows: Click Calculate Now under the category Calculation on the Formulas (or Home)tab (or press F9). Every calculation in every open workbook in Excel is recalculated.
When Excel is in manual calculation mode, it only recalculates open workbooks upon your request (by hitting F9 or Ctrl+Alt+F9) or when you save a worksheet. To prevent a lag when making changes, you must switch computation to manual mode for workbooks that take longer than a nanosecond to recalculate.
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