The factorial of a number is the product of all numbers less than n. For example, the factorial of four would be four x three x two x one, which amounts to 24. You can find the factorial of number in Excel 2010/13 with the FACT function.
Open your Excel spreadsheet, and enter the number you want to find the factorial for in a cell. Then select a cell to add the function to. Click the fx button to open the window in shot below.
Select FACT from that window. That will open the window shown below. Press the button beside the text, and then select the cell that includes the number to find the factorial for it.
Press the OK button to close the window. The function will then show you the factorial for the selected number as below.
Another way you can find the factorial of a number is to select a cell and then type =FACT in the function bar. Beside that enter the number you want to find the factorial for in brackets. For example, you could enter =FACT(4) and press Enter. The cell will then include the factorial for that number.
With that function you can now find the factorial for a variety of numbers in Excel 2010/13 spreadsheets. This YouTube video  also shows you how to find factorials with Excel.