Spill Resistant 2 Gallon Smart Funnel prevents overfilling of drum barrels. The Secret: With the funnel extension, trapped air stops the liquid from filling to the top of the barrel. It leaves just enough room to add 2 gallons more to the barrel, which is the capacity of the E-Z SMART DRUM FUNNEL. In order for the Smart Funnel to work properly, it must be removed immediately when fluid no longer flows freely . This means the drum is full and cannot take any more fluid.