This article describes the formula syntax and usage of the ATAN function in Microsoft Excel.
Description
Returns the arctangent, or inverse tangent, of a number. The arctangent is the angle whose tangent is number. The returned angle is given in radians in the range pi/2 to pi/2.
Syntax
ATAN(number)
The ATAN function syntax has the following arguments:

Number Required. The tangent of the angle you want.
Remark
To express the arctangent in degrees, multiply the result by 180/PI( ) or use the DEGREES function.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Formula 
Description 
Result 
=ATAN(1) 
Arctangent of 1 in radians, pi/4 
0.785398163 
=ATAN(1)*180/PI() 
Arctangent of 1 in degrees 
45 
=DEGREES(ATAN(1)) 
Arctangent of 1 in degrees 
45 