As far as I know, excel calculates to 15 sig fig
From excel Help
15 digit precision Excel stores and calculates with 15 significant digits of precision
Excel calculates stored, not displayed, values The displayed, and printed, value depends on how you choose to format and display the stored value
Precision of calculations: When a formula performs calculations, Excel usually uses the values stored in cells referenced by the formula. For example, if two cells each contain the value 10.005 and the cells are formatted to display values in currency format, the value $10.01 is displayed in each cell. If you add the two cells together, the result is $20.01 because Excel adds the stored values 10.005 and 10.005, not the displayed values. You can change the precision of calculations so that Excel uses the displayed value instead of the stored value when it recalculates formulas
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Geof