The Arccotangent is the inverse of the cotangent.
Therefore, for the simple rightangled triangle below, the arccotangent of the adjacent side, a, divided by the opposite side, o, is equal to the angle θ.
I.e. for the triangle above,
The Inverse trig. functions are discussed further on the Wikipedia Inverse Trigonometric Functions PageThe Excel Acot function calculates the arccotangent (i.e. the inverse cotangent) of a given number, and returns an angle, in radians, between 0 and π.
Note: the Acot function was only introduced in Excel 2013 and so is not available in earlier versions of Excel.
The syntax of the function is:
Where the number argument is the value that you want to calculate the arccotangent of.
Converting from Radians to Degrees
If you want the angle returned by the Acot function to be expressed in degrees, you can convert it, using the Excel Degrees function:
The following spreadsheet shows the Excel Acot Function, used to calculate the arccotangent of four different values:
Formulas:
 Results:

Note that, in cell A4 of the above spreadsheet, the Excel Degrees function is used to convert the function result from radians into degrees.
Further details and examples of the Excel Acot function are provided on the Microsoft Office website.
If you get an error from the Excel Acot function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if the supplied number is not recognised as a numeric value. 