To determine the dimensions of the pond liner you will require for the size of pond you want, you need to use another formula.
Size of pond desired = Length x Width x Depth
Size of pond liner required for desired pond = ( Length + (2 x depth)) x (width + (2 x depth))
(The reason doubling the depth and adding to both length and width is to allow enough liner to travel not only across the length/width of the pond, but also up the sides!)
In example: A pond 7' Long by 17' Wide by 3' Deep - you will need
7 + (2 x 3) by 17 + (2 x 3) which equals a 13' x 23' liner.
It is then advisable to take this measurement and add a two feet to both the length and width to allow at least a 1 foot overlap around the edge of the pond, so that it may be properly bedded in for coping stones used and so on. It also allows for slight discrepancies to occur in the dimensions of the pond without leaving you short of liner and having the possibility of the pond over-spilling. This advice used with the example shown above leads us to select a 15' x 25' liner.