I have a subform of invoice items held in a main form of invoices. 

When the form is loaded, all of the items (ItemCosts) are added up and the figure is loaded in the SubTotal field of the main form using:

input.SubTotal = Invoice_Items[InvoiceID == input.ID].sum(ItemCost);

This code will not help with any changes made to the open form using 'InvoiceItems > ItemCost > On User Input' as the code is working on the main database and not the open form. 

How do I access (and sum) all the items in the subform before they are submitted.

Thanks,

Guy