Is there a way that I can create an assembly for an item that totals the volumes automatically?

Here's the situation - I need to quanitfy a footing by the volume of rebar needed + the volume of concrete needed. I can create the assembly and set the two volumes by hand (ie if I have a 20 cu ft footing and I know there is 2 cu ft or rebar - I can input the rebar and concrete independantly) but I'd like to be able to input the rebar volume and have the concrete volume automatically calculate the overall volume - rebar volume.