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Drainage and venting fixture unit values

Find information about how plumbing loading works within the Plumbing Code. For example, if you are adding a bathroom to your house, you can see whether an existing drain pipe is large enough.


Drainage Fixture Unit (DFU) Values for Typical Household Fixtures (excerpt from Uniform Plumbing Code Table 702.1)

Individual fixtures Minimum size trap and trap armb Number of units
 Bar sink 1-1/2 inch  1.0
 Bathtub or combination bath/shower  1-1/2 inch  2.0
 Clothes washer, domestic, 2-inch standpipe  2 inch  3.0
 Dishwasher, domestic with independent drain  1-1/2 incha  2.0
 Kitchen sink, domestic, with or without food waste grinder and/or dishwasher  1-1/2 incha  2.0
 Laundry sink, 1 or 2 compartments  1-1/2 incha  2.0
 Lavatory (wash basin)  1-1/4 inch  1.0
 Shower stall  2 inch  2.0
 Water closet, 1.6 GPF gravity tank  3 inch  3.0

a Provide a 2-inch (51-mm) minimum branch drain beyond the trap arm.

b Trap sizes must not be increased to the point where the fixture discharge may be inadequate to maintain their self-scouring properties.

Maximum unit loading and maximum length of drainage and vent piping

 Size of pipe, inches  1-1-4  1-1/2  2  2-1/2  3  4
Maximum units
Drainage pipinga vertical 1 2b 16c 32c 48d 256
Horizontal 1 1 8c 14c 35d 216c
Maximum length
Drainage piping vertical, feet 45 65 85 148 212 300
Horizontal (unlimited)            
Vent piping (see note)
Horizontal and vertical maximum units 1 8c 24 48 84 256
Maximum length, feet 45 60 120 180 212 300

a Excluding trap arm

b Except sink, urinals and dishwashers

c Except 6-unit traps or water closets

d Only 4 water closets or 6-unit traps allowed on any vertical pipe or stack; and not exceed 3 water closets or 6-unit traps on any horizon branch or drain.

Note: The diameter of an individual vent must not be less than 1-1/4 inches nor less than 1/2 the diameter of the drain to which it is connected. Fixture unit load values for drainage and vent piping must be computed from the above table.

No more than 1/3 of the total permitted length of any vent may be installed in a horizontal position. When vents are increased 1 pipe size for their entire length, the maximum length limitations specified in this table to not apply.