Plumbing Vent Size & DFU Airflow Calculator
Determine minimum plumbing vent pipe diameter (1.5", 2", 3"), Drainage Fixture Units (DFU), and maximum developed pipe length per UPC/IPC codes.
Calculated Results
Frequently Asked Questions
What is a Drainage Fixture Unit (DFU) in plumbing?
A Drainage Fixture Unit (DFU) is a mathematical measure of the probable discharge flow rate into the drainage system by various plumbing fixtures. A bathroom sink is 1 DFU; a shower is 2 DFU; a residential water closet (toilet) is 3 to 4 DFU.
Why does plumbing need vents?
Plumbing vents introduce atmospheric air behind draining water. Without vents, the rush of draining wastewater creates a vacuum (negative pressure) that siphons water out of P-traps, allowing toxic, combustible sewer gases (methane, hydrogen sulfide) into the home.
What is the golden rule for sizing vent pipes?
Under the Uniform Plumbing Code (UPC 904.1), a vent pipe cannot be smaller than 1-1/4 inches, and cannot be smaller than one-half the diameter of the drain pipe it serves (e.g. a 3-inch toilet drain requires a minimum 1-1/2-inch or 2-inch vent).
Why do vent pipes increase in diameter before penetrating the roof in cold climates?
In northern climates prone to freezing, warm moist air inside the plumbing vent can form thick hoarfrost ice at the roof penetration. Codes require upsizing vent pipes to a minimum 3-inch diameter at least 1 foot below the roofline to prevent ice blockage.