As fellow Access MVP John Vinson likes to say "Storing calculated data
generally accomplishes only three things: it wastes disk space, it wastes
time (a disk fetch is much slower than almost any reasonable calculation),
and it risks data validity, since once it's stored in a table either the
Total or one of the fields that goes into the total may be changed, making
the value WRONG."

One approach for "how" is to have 2 fields on the form: one visible, and one
hidden. Bind the hidden field to the appropriate field in the form. In the
form's BeforeUpdate event, put code to transfer the value that's in the
visible field to the hidden field.

In addition to Doug's method, you can write a few lines of code that check
for  the existence of the other elements of the calculation then fills the
the calculation in the bound text box.

If Not IsNull(Me.txtField1) Then
   Me.txtField3 = Me.txtField2 * Me.txtField1
End If

Doug already told you why not to. Good database design is important for a
smoothly running application.