For the simple rightangled triangle below, the cosine of the angle θ, is the ratio of the adjacent side, a, to the hypotenuse, h.
i.e. for the triangle above,
The Excel COS function calculates the cosine of a given angle (which is supplied in radians).
The format of the function is :
Where the number argument is the angle, in radians, that you want to calculate the cosine of.
Converting from Radians to Degrees
If your angle is in degrees, you will need to convert it into radians before supplying it to the cos function.
The easiest way to do this is to use the Excel Radians function:
Column B of the spreadsheet below, shows examples of the Excel Cos Function, used to calculate the cosine of different angles, which are in either degrees or radians.
The formulas are shown in the spreadsheet on the left and the results are shown in the spreadsheet on the right.
Formulas:

Results:

Further information and examples of the Excel Cos function can be found on the Microsoft Office website.