Automatically Numbering Rows

Libby wants to insert a row number into column A of her worksheet. She wants the column to reflect the correct number of the row, even when she adds or deletes rows.

There are any number of formulas you can use in column A that will return a row number. Perhaps the easiest is to use the ROW function, like this:

=ROW()

This formula returns the row number of the cell in which the formula appears. If you want to offset the row number returned (for instance, if you have some headers in rows 1 and 2 and you want cell A3 to return a row value of 1), then you can modify the formula to reflect the desired adjustment:

=ROW()-2

Of course, the ROW function isn't the only candidate for your formulas. You can also use a formula that actually examines the contents of the adjacent column (B) and return a row number only if there is something in that adjacent cell.

=IF(TRIM(B1)<>"",COUNTA($B$1:B1)&".","")

This formula, placed in cell A1, examines the contents of cell B1. If there is something there, then the COUNTA function is used to count the number of occupied cells between cell B1 and whatever cell is to the right of where this formula is placed. The formula also places a period after the row number that is returned. Make sure the dollar signs are included, as shown, and then copy the formula down as many cells as necessary to create your row numbers.

The advantage to a formula such as this one is that it checks to see if something is in column B before it returns a row number. This means that you can copy the formula down beyond the actual end of your data rows, and only those rows that have data (triggered by something in column B) will have a row number. The same sort of technique could be used with the ROW function instead of the COUNTA function:

=IF(TRIM(B1)<>"",ROW()&".","") 

Source  : http://excelribbon.tips.net