The acosh function returns the inverse hyperbolic cosine. Therefore,
The equation used to calculate the inverse hyperbolic cosine is:
Further information on the inverse hyperbolic functions can be found on WikipediaThe Excel Acosh function calculates the inverse hyperbolic cosine of a supplied number.
The syntax of the function is:
Where the number argument is the positive real number that you want to calculate the inverse hyperbolic cosine of. This value must be greater than or equal to 1.
The following spreadsheet shows the Excel Acosh Function, used to calculate the inverse hyperbolic cosine of three different values:
Formulas:
 Results:

For further details and examples of the Excel Acosh function, see the Microsoft Office website.
If you get an error from the Excel Acosh Function, this is likely to be one of the following:
#NUM!    Occurs if the supplied number is less than 1. 
#VALUE!    Occurs if the supplied number argument is not recognized as a numeric value. 