Excel has builtin trigonometric functions that make it easy to find the cosine, sine, and tangent of a rightangle triangle — a triangle containing an angle equal to 90 degrees.
The only problem is that these functions require angles expressed in radians, not degrees. While radians, a unit of angles, are a legitimate way of measuring angles based on the radius of a circle, they are not something most people work with on a regular basis.
To help the average spreadsheet user get around this problem, Excel has the RADIANS function, which makes it easy to convert degrees to radians.
These instructions apply to Excel 2019, 2016, 2013, 2010, and Excel for Microsoft 365.
RADIANS Function Syntax and Arguments
A function's syntax refers to the layout of the function and includes the function's name, brackets, and arguments.
The syntax for the RADIANS function is:
=RADIANS(Angle)
The Angle argument is the angle in degrees you want to convert to radians; it can be entered as degrees or as a cell reference to the location of this data in your Excel worksheet.
Excel RADIANS Function Example
This example uses the RADIANS function to convert a 45degree angle to radians. The information covers the steps used to enter the RADIANS function into cell B2 of the example worksheet.
=RADIANS(A2)
=RADIANS(45)
Options for entering the function include:
 Typing the complete function into cell B2, as shown in cell C3 above
 Selecting the function and its arguments using the Function Dialog Box
Although it is possible to enter the complete function manually, many people find it easier to use the Function Dialog Box, as it takes care of inputting the function's syntax such as brackets and comma separators between arguments.
Using the Function Box (Formula Builder on Mac) for RADIANS

Click on cell B2 in the worksheet — this is where the function will go.

Click on the Formulas tab of the ribbon menu.

Choose Math & Trig from the ribbon to open the function dropdown list.

Click on RADIANS in the list to bring up the Function Dialog Box.

Click on the Angle line.

Click on Cell A2 in the worksheet to enter the cell reference as the function's argument.

Click Done to complete the function — the answer 0.785398163, which is 45degrees expressed in radians, appears in cell B2.

If your worksheet shows fewer numbers to the right of the decimal point, you can format the cell to display more.
You can input data directly into the dialog box rather than cell references, but that makes it harder to update formulas and functions.
Using PI() Function to Get the Angle in Radians
An alternative, as shown in cell C4 of the example image, is to multiply the angle by the PI() function and then divide the result by 180 to get the angle in radians.
It looks like this:
=A2*PI()/180