Rob H,
In your table... you don't.
Since you have the GrossSales and the TotalCosts in your table, then you
can
always calculate NetSales "on the fly" in any form, query, or report.
= GrossSales - TotalCosts
As a general normalization rule... don't store any values in a table,
that can be
derived from data you already have.
--
hth
Al Campagna
Microsoft Access MVP 2007-2009
http://home.comcast.net/~cccsolutions/index.html

"Find a job that you love... and you'll never work a day in your life."

"Rob H" wrote in message
...
I have a table with a Gross Sales field and a Total Costs field, there is a
third field called Net Sales. How can I create a function to take Gross
Sales minus Total Costs to populate the Net Sales field? All three are
set
to currency.