Sanitary Piping - Drainage Fixtures Unit Loads

Maximum Drainage Fixture Unit (DFU) loads in sanitary piping systems. A Fixture Unit is equal to one cubic foot of water drained in a 1 1/4 inch pipe over one minute (≈ 7.48 U.S. gpm, ≈ 0.47 l/s).

Pipe Size (inches)Pipe Size (mm)Horizontal Branch Max DFUStack < 3 Stories Max DFUStack > 3 Stories Max DFUTotal for StackTotal for One Story
1 1/2403482
250610246
2 1/2651220429
38020487220
410016024050090
51253605401100200
61506209601900350
Pipe Size (inches)Pipe Size (mm)1/4 in/ft (2.1 cm/m) Slope Max DFU1/2 in/ft (4.2 cm/m) Slope Max DFU
2502126
2 1/2652431
3804250
4100216250
5125480575
61508401000

Notes

  • Fixture Unit Definition: A fixture unit is equal to one cubic foot of water drained in a 1 1/4 inch pipe over one minute (≈ 7.48 U.S. gpm, ≈ 0.47 l/s)
  • Stacks: Vertical runs of plumbing waste or drainage lines
  • Water Closet Restrictions:
    • 1 1/2", 2", 2 1/2" pipe sizes: No water closet permitted
    • 3" pipe size: Maximum 2 water closets
    • 3" and larger stacks (>3 stories): Maximum 6 water closets

Tables

Table 1: Drainage Fixture Units (DFU) in Stacks and Horizontal Fixture Branches

This table shows the maximum DFU values for horizontal fixture branches and various stack configurations.

Table 2: Drainage Fixture Units (DFU) in Building Drains and Building Drain Branches from Stacks

This table shows the maximum DFU values based on pipe slope in building drains and drain branches from stacks.

All values are based on the major U.S. plumbing codes.