Autosum after Insert Row

I only use Excel when I really have to so excuse this question if it's missing some basic understanding. I can get into VBA and macros if necessary, but it hurts. The actual workbook is much more complicated than the following simplified problem, but the principle is the same:

A column has 11 rows of numbers and in the 11th row is an autosum =SUM(A1:A10). Standard stuff. However, when a new row is inserted at row 11, the new 12th row should now read =SUM(A1:A11) but it remains as it was, to A10.

I would have thought it logical for Excel to update the row count automatically, but it doesn't. Is there a simple way to do this or using VBA if absolutely necessary?

Thanks.