The Arccosine is the inverse of the cosine.
Therefore, for the simple rightangled triangle below, the arccosine of the adjacent side, a, divided by the hypotenuse, h, is equal to the angle θ.
I.e. for the triangle above,
The Excel ACOS function calculates the arccosine (i.e. the inverse cosine) of a given number, and returns an angle, in radians, between 0 and π.
The syntax of the function is:
Where the number argument is the value, between 1 and +1, for which you want to calculate the arccosine.
Converting from Radians to Degrees
If you want the angle returned by the Acos function to be expressed in degrees, you can convert it, using the Excel Degrees function:
The following spreadsheet shows the Excel Acos Function, used to calculate the arccosine of four different values.
Formulas:

Results:

Note that in cell A4 of the above spreadsheet, the Excel Degrees function is used to convert the calculated angle from radians to degrees.
For further details and examples of the Excel Acos function, see the Microsoft Office website.
If you get an error from the Excel Acos function, this is likely to be one of the following:
#NUM!    Occurs if the supplied number argument is outside the range 1 to +1. 
#VALUE!    Occurs if the supplied number argument is nonnumeric. 