summing quantities

I'm creating an order form which can accept products in different unit bases. For example, a single order may contain products that are sold by EACH unit, products that are sold by the DOZEN, and products that are sold by the SET. So, my products have a field called UnitType that states whether it is sold by EACH, DOZEN or SET. In the order form, there is a field for product no, quantity, unit type, unit price and subtotal. Summing the subtotal to find the grandtotal is simple using "=Sum([subtotal])", however I want to sum the quantities separately- by EACH, DOZEN and SET. Then eventually paste them in the order form and report. Any suggestions? Thanks