Besides forms and reports, you can also do your calculations in a query.
However, BEWARE: Access is NOT Excel. You cannot do calculations inside of a field in a table.
Excel allows you to do ranged calculations that span a subset of all rows in a sheet. Without another way to identify participants/contributors to the computation, this class of action is not possible in Access. This is because Excel identifies cells by sheet, row, column. Access identifies things by tables, fields and records. The big difference? Record number is not a constant because of variable sort order abilities