Place a text box in the Form Footer section of the subform.
Set its Control Source to sum the field you want, e.g.:
=Sum([Qty])
If the subform is in Form or Continuous view, you can see the total. In
Datasheet view you can't (though there is a way in Access 2007), so you will
need a text box on the main form to display the total. Its Control Source
will be like this:
=[Sub1].[Form]![Text0]
where you use your subform control name instead of Sub1, and the name of the
text box created above instead of Text0. The .Form bit stays as it, as
explained here:
http://allenbrowne.com/casu-04.html
For an example of such a form, look at the Orders form in the Northwind
sample database.
There is an extra wrinkle if new records cannot be added to the subform. Its
Detail section goes completely blank when there are no records to show, and
the expressions error out. Post back if you hit this.
