Drain System Pipe Sizing

Sizing Terms:
  • fixture units,
  • Canadian gallons per minute flow,
  • hydraulic load.
Fixture unit
means the unit based on the rate of discharge,
time of operation and frequency of use
of a fixture.

Canadian Gallon per Minute Flow:
The GPM  measures the number of gallons that pass through a pipe
of a given diameter at a given slope.

Hydraulic Load
is the number of fixture units discharging into the drainage system.
Table 7.4.9.3.
Sizing 1 1/4" to 2" Drainage Piping:
(based on fixture units)
  1. Table 7.4.9.3. - listed fixtures,
  2. Table 7.4.10.2. - unlisted fixtures
Sizing Horizontal Drainage Piping:
when sizing horizontal drainage piping there are two sections of the drainage system that must be sized, horizontal waste pipes and horizontal branches.

Sizing Horizontal Waste Pipes:
use Table 7.4.9.3. that lists
  • hydraulic load of a fixture or appliance,
  • minimum permitted size of a fixtures outlet pipe that denotes:
  • fixture trap size,
  • fixture drain size,
  • fixture waste pipe size.
No reduction in pipe size may be made.

Sizing Horizontal Branches of 1 1/4", 1 1/2" and 2":
Use Table 7.4.10.3.A..

Steps to size horizontal branches:
  1. Determine the number of fixture units carried by each waste pipe connected to the horizontal branch.
  2. Starting at two most upstream waste pipes add together the fixture units or hydraulic loads from each waste pipe.
  3. With the hydraulic load determined go to Table 7.4.10.3.A. and size the horizontal branch.
The horizontal branch may never be smaller than
any waste pipe discharging into it.
  1. When the total hydraulic load exceeds the values given for each pipe size in Table 7.4.10.3. the next larger pipe size must be used.
  2. Size the next waste pipe connection into the horizontal branch.
Any horizontal branch carrying more than 6 fixture units
must be a minimum of 3" in size.

1 1/4", 1 1/2" and 2" Soil and Waste Stacks Sizing :
(based on fixture units)
  1. Sizing the stack starts at the highest horizontal branch or waste pipe connected to the stack.
  2. The highest section of the stack will be the same size as the largest horizontal branch or waste pipe discharging into it.
  3. The stack is sized at every waste pipe or horizontal branch connection.
    Table 7.4.10.5.A.
  4. If the overall height of the stack is 3 storeys or less column 2 is used. If the stack is more than 3 storeys in height column 3 is used. Column 4 is used to limit the number of fixture units that may discharge at any floor level into a stack which is higher than 3 storeys.
  5. When the total hydraulic load exceeds the values for each given pipe size in any column of Table 7.4.10.35.A. the next larger pipe size must be used.
  6. The entire stack is sized from top down by repeating steps 3 and 4.
The horizontal Drainage installed at the base of the stack is sized as a horizontal branch
(see picture).

3" Drainage Pipe Sizing:
horizontal: 3" branch may have up to 2 water closets,  if a 3rd W.C. discharges into the branch, the size must increase to 4" where the 3rd W. C. connects;

fixture units from each waste pipe or horizontal branch must be added to the cumulative fixture units total that is then converted into GPM (Gallons Per Minute Flow) using expanded Table 7.4.10.3. B. in the Appendix. Table 7.4.10.3.C. is used to size  horizontal piping.

stack: 3" stack may have up to 6 water closets either flush tank type or direct flush discharging into it. If a 7th W. C. connects to the soil stack, the size will have to become 4", where the 7th W. C. connects.

 Convert hydraulic load in fixture units for each waste pipe and horizontal branch into GPM, beginning from the top down size a soil or waste stack using Table 7.4.10.5.B. At the point of connecting the next waste or horizontal branch add new hydraulic load in fixture units  to the old one. Convert into GPM. Size the stack.

GPMs must never be added together.

building drain: soil or waste stacks terminating as a horizontal branch or the building drain is sized like usual horizontal piping using Table 7.4.10.3.C., when fixture units from upstream pipes horizontal and vertical are added together and converted to GPM using Table 7.4.10.3.B.

Every sanitary building drain and every sanitary building sewer
shall be at least 4" size.


Now go to



7.4.10.3. Fixture Loading for Horizontal Drain

  1. No horizontal sanitary drainage pipe of less than 3" size shall a fixture loading in excess of that permitted by Table 7.4.10.3.A.
  2. Reserved
  3. The horizontal sanitary drainage pipe size shall be that size determined from Table 7.4.10.3.C. after the total connected load in fixture units on a horizontal sanitary drainage pipe is converted to gallons per minute in accordance with Table 7.4.10.3.B.
  4. Horizontal sanitary drainage pipe shall be designed to carry no more than 65% of its full capacity.

7.4.10.5. Hydraulic Loads for Vertical Drains

  1. No vertical waste pipe, branch or stack of less than 3" diameter shall have a hydraulic load in excess that permitted by Table 7.4.10.5.A.
  2. Reserved.
  3. The vertical sanitary drainage pipe size shall be that size determined from Table 7.4.10.3.B. after the total connected load in fixture units on a vertical drainage pipe is converted to gallons per minute in accordance with Table 7.4.10.3.B.
  4. Vertical sanitary drainage pipe shall be designed to carry no more than 33% of its full capacity.