Calculation in subform

Calculation in subform

Hi everyone.

I have my subform as with an struture of a product, and I need to calculate in this way:

Inside of the form "MODELOS" I have subform "ESTRUTURA"
That means that I have a field "Consumo_Unit" that means consume, and I have a field "Custo" that is the total cost, and I also have a field with name "Custo_Unit1" this mean that I need this calculation:

"Consumo_Unit" * "Custo" = "Custo_Unit1"

So, how I can do this ?
Thank in advance.