Example:  If your container is

First:

Find the Perimeter [P = 2 x (L +W)] and add 4".

Second: 

Estimate the Length of the Liner:

First:

Perimeter  =   2 x (6"D + 12"W)  and add 4

                =   2 x (18) and add 4

                =   36             +      4

                =   40

Second: 

Length       = 15" L + 1/2(6"D) +  4" (overhang)

                  = 15"    +      3"       + 4"

                  =  22" 

You need a liner that is approx. 40" Perimeter and 22" L.  Example that fit this requirement is: