Weighted Moving Average

I am trying to produce a weighted moving average on closing stock prices. Their is only one column of data which contains the closing price. I want to have a 7 period weighted moving average of the price. Currently I have to write the following formula:

((7*P1)+(6*P2)+(5*P3)+(4*P4)+(3*P5)+(2*P6)+P7)/28

- where P7 is the price 7 periods ago, P6 is the price 6 periods ago...and so on.

The problem is if I want to look at a 3 period or 21 period weighted moving average I have to re-write the entire formula. Instead I am trying to find a formula that would look at a cell for the number of periods weighted moving average that I want and produce my results.

Is this possible?

Thanks

This is a brilliant formula Daddy Long Legs. How do I adjust it to calc. WMA in the other direction as my data is going from newest to oldest not oldest to newest.

Thank you.