In B2 =AVERAGE($A2,B$1)
Copy down

"Trout" wrote:

I want to build a matrix of values where each cell is the average of its
column header and row header. For example: I want cell B2 to be the average
of A2 and B1, C2 to be the average of A2 and C1 and B3 to be the average of
A3 and B1. No matter how I enter the equation and try and drag to autofill
Excel doesnt understand what I am trying to do. Any tips?