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Plumbing & Fire Calculation guide Calculate a value

Sizing drainage pipe by fixture units rather than by flow

Why drainage is sized by a loading unit, and reading the code table to get the building drain.

The engineering question

There is no measured flow rate for a building drain. How do you choose the diameter?

Worked example

The figures below were produced by running the Drainage Fixture Unit Calculator with the same inputs — during the build, from its authoritative engine.

Results
Total drainage load 90 DFU
Minimum building drain 4 in
Minimum WC branch 3 in
Code basis IPC 2018
Assessment Total drainage load 90 DFU under IPC 2018 → building drain/sanitary main ≥ 4".
Venting note Vent sizing follows the same DFU load at 3 ft/min (IPC Table 711). A 3" soil branch carries a WC; the vent shall be at least ½ the drain diameter.
  1. Each fixture contributes a loading unit defined by the code.
  2. Add them to get the total drainage load.
  3. Look the total up in the horizontal drain sizing table for the minimum building drain.

Sanity check

IPC and UPC give different values for the same fixtures. Always state which code the drawing was produced against.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

References & standards

  • IPC 2018 Table 706.1 — fixture unit values
  • IPC 2018 Table 710.1(1) — building drain sizing
  • UPC 2021 Tables 702.1 / 703.1

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