The Arctangent is the inverse of the tangent.
Therefore, for the simple rightangled triangle below, the arctangent of the opposite side, o, divided by the adjacent side, a, 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 ATAN function calculates the arctangent (i.e. the inverse tangent) of a given number, and returns an angle, in radians, between π/2 and +π/2.
The syntax of the function is:
Where the number argument is the value, for which you want to calculate the arctangent.
Converting from Radians to Degrees
If you want the angle returned by the Atan function to be expressed in degrees, you can convert it, using the Excel Degrees function:
Column A of the following spreadsheet shows the Excel Atan Function, used to calculate the arctangent of four different values.
Formulas:
 Results:

Note that in cell A4, the Excel Degrees function is used to convert the calculated angle from radians to degrees.
Further information and examples of the Excel Atan function can be found on the Microsoft Office website.
If you get an error from the Excel Atan Function, this is likely to be the #VALUE! error:
#VALUE!    Occurs if the supplied number argument is nonnumeric. 