i have created an invoice and i want to create a sum of all the individual costs which are generated to form grand total
i tried sum([total cost]) but it doesnt work [total cost] is the field i am adding up

Presuming total cost is a calculated control on the report, you can't sum it directly:

http://support.microsoft.com/kb/207763

That said, it sounds like you may have fields for each costs, which typically would be a normalization mistake.

ok ive tried doing the solutions but every time it still have #Error
because i want to invoice the customer so it displays all the products and individual price but totals them all up to get a final amount

Have you got your totals field in a group footer as opposed to being in the detail section?
Assuming you have group by footer and header based on the invoice number.

Can you post the db?

i did have it in the footer rather than the detail cheers works

ahh sorry theres a problem if i do have more than one product it put the total twice as in for each product which is displayed

Database received is password protected.

oh forgot about that only 123 for time being

Sorry, I lost track of this thread. Did you get it resolved? If not, the only thing wrong I see right off is that the textbox with the total should be in the group footer, not the detail section. You currently aren't displaying that section, but you can change that in the Sorting and Grouping area.