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 (ie. the inverse cosine) of a given number, and returns an angle, in radians, between 0 and π.
The format 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:
Column A of the spreadsheet below, shows examples of the Excel Acos Function, used to calculate the arccosine of different values. In the example in cell A4, the result is converted from radians, into degrees.
Formulas:

Results:

Further information and examples of the Excel Acos function can be found on the Microsoft Office website.
If you get an error from the Excel Acos function, this is likely to be the #NUM! error:
#NUM!    Occurs if the supplied number argument is outside of the range 1 to +1. 