The Arcsine is the inverse of the sine.
Therefore, for the simple rightangled triangle below, the arcsine of the opposite side, o, divided by the hypotenuse, h, is equal to the angle θ.
i.e. for the triangle above,
The Excel ASIN function calculates the arcsine (ie. the inverse sine) 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, between 1 and +1, for which you want to calculate the arcsine.
Converting from Radians to Degrees
If you want the angle returned by the Asin 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 Asin Function, used to calculate the arcsine of different values. In the example in cell A4, the result is converted from radians, into degrees.
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 Asin function can be found on the Microsoft Office website.
If you get an error from the Excel Asin Function, this is likely to be the #NUM! error:
#NUM!    Occurs if the supplied number argument is not in the range 1 to +1. 