Calculations on a form

[JL]

I would put the calculation in a textbox.

For starters do I put the calculation in the rowsource of
the combobox or does this go elsewhere? and can this be
done in a textbox instead.

Unless there is a text/combo box on the form
called "productweights.weight". It will be null.
Forms!Form1!Combo1, if this is on the same form, you can
refer to it by using this convention. "Me![Combo1]".

For seconds would something along these lines work:
=([productweights.weight]/100)*(Forms!Form1!Combo1)

Hope this will help.