forecasting item number by year

Looked at your dB. It appears that you are trying to do your calculation in the table. In Access, you do not, I reapeat, do not do calculations in tables. Tables are for storing the raw data. Access is not an extension of Excel. It is a relational database and you need to not think in terms of spreadsheets. Having said all that, calculations are performed in Queries and sometimes in Forms. Not knowing how you will be using this or even publishing the information, it is probably best to do your calculations in your queries. Additionally, I noticed that there was no form associated with the table. It is not a good idea to enter data directly into tables. You should build forms and enter the data through the form. I know this is a bit of rant, but dB that are not built properly will give you lots of anxiety and issues in the future.

Alan