Finding Weighted Average Rate without updating source

Hi! I am trying to find out Weighted Average Rates (W.A.R) of some loans divided into categories using a Pivot Table. I have been looking everywhere but can't seem to find an answer that DOESN'T use a helper column created in the source table to find (weight *rate). but What if you can't update the source table? I have tried to add a calculated field in the pivot table with the W.A.R. formula - SUMPRODUCT(rate,weight)/SUM(weight) but it returns zeros. I have tried adding a 'helper field' using the Calculated Field option but that doesn't work either, anything I do, the calculated field returns zeros only. Please help, Thanks!