The Excel FLOOR function rounds a supplied number towards zero to the nearest multiple of a specified significance.
The syntax of the function is:
Where the arguments are as follows:
number    The initial number that is to be rounded 
significance   
The multiple of significance that the supplied number is to be rounded to. (should generally have the same arithmetic sign (positive or negitive) as the supplied number argument) 
Note: In Excel 2003 & 2007, the significance argument must have the same arithmetic sign (positive or negative) as the initial number argument. However, in Excel 2010, the Floor function has been improved, so that it can now handle a negative number argument and a positive significance argument. In this case, the Floor function reverses the direction of the rounding (ie. rounds the supplied number away from zero).
The following spreadsheet shows the Excel Floor function used with several different significance values, on the positive and negative numbers 26.75 and 26.75.
The functions are shown in the spreadsheet on the left and the results are shown in the spreadsheet on the right.
Formulas:

Results:

The above examples show how, when the number and the significance arguments have the same arithmetic sign, the Floor function rounds the supplied number towards zero.
Different combinations of positive and negative arguments, are shown in the Excel 2010 spreadsheet below.
Formulas:

Results:

This example shows how:
Further information on the Excel Floor Function is provided on the Microsoft Office website.
If you get an error from the Excel Floor function, this is likely to be one of the following :
#NUM!   
Occurs :


#DIV/0!    Occurs if the supplied significance argument = 0  
#VALUE!    Occurs if either of the supplied arguments is nonnumeric 