In rows I have inventory tracked weekly. I am trying create a formula
that will give me an average of my last 12 weeks. Such that when I
add a new column for the new weeks inventory level the formula will
give the average of that week and the last 11.

example:

a1 b1 c1 d1 e1 f1 g1 h1 i1 j1 k1 l1 12 week
average (a1:l1)
1 2 3 4 5 6 7 8 9 10 11 12
6.5

next week

a1 b1 c1 d1 e1 f1 g1 h1 i1 j1 k1 l1 m1 12 week
average (b1:m1)
1 2 3 4 5 6 7 8 9 10 11 12 13 7.5


I have tried working with the following formula but have had no
luck.

=AVERAGE(OFFSET(A1,,COUNT(A1:L1)-12,,12))

Any help would be appreciated. Thanks