Joanne,
Well, to be accurate here, forms do not have fields. Fields are in
tables (and queries), and forms can have controls that serve as a
convenient way of viewing the data in the fields. Normally it would be
somewhat pedantic of me to point this out, but in this case I think it
is relevant to your question. If a value can be calculated from, or
derived from, existing data, then it is very unlikely that you want a
second field based on the value of the first. This would almost
certainly be an invalid database design. On the other hand, it is often
a legitimate requirement to have a control on a form to *display* (as
distinct from store) a derived or looked-up value. This is normally
done via either a calculated field in the query that the form is based
on, or via an expression in the Control Source property of an unbound
textbox on the form. So, sorry can't be more explicit without more
details of what you want... if you can post back with a specific
example, then someone may be able to advise more precisely.