Sumifs across relative numbers of columns based on cell value.

So I'm working with a forecast for individual items. I need a way to enter a number into a cell (named 'timeframe'), and for the sum of a list of items to populate based on the 'timeframe' number of weeks (column).

Each column is a week of data, and if 8 week is selected from a data validation, I need the first 8 columns to be sumif'd based on item numbers.

I'm fairly sure it's going to involve some combo index / sumifs, and would like it to be fairly flexible to allow new items and columns to be added..

Appreciate anyone who can help me with this.

Correct!

A sum of demand by item, based on the amount of weeks I want to look at.

Worksheet "dash"

B4=SUMPRODUCT((fcst!$A$2:$A$40=A4)*(fcst!$B$1:$P$1="Week "&$B$1)*(fcst!$B$2:$P$40)) copydown