Thanks for the sample file. But you have not mentioned the details of your forcasting process. Excel has lot of statistical formula, but your process need to be understood to link it to available forumla. This forum may not have statisticians who can suggest you with a few options. Either provide the normal statistical formula you may like to use in a manual calculation or go through the list of statistical formula support by Excel. You can view a list of formula by using Formulas->More Functions->Statistical. Review of these formula may suggest a process to select. You can then raise fresh query providing the formula in which you may need any help.