What this calculator does
Drainage pipe is not sized by volume of flow; it is sized by a loading unit that each fixture contributes. This calculator counts your fixtures, totals the units and returns the minimum building drain, which is the number that appears on almost every drainage drawing.
How to use it
- Enter the number of each fixture type in the building.
- Choose the code that applies in your jurisdiction — IPC and UPC assign slightly different unit values.
- The total determines the building drain and sanitary main.
- Vents and branches have their own tables; the WC branch is always at least 3 inches.
Governing formula
Σ DFU = Σ (n_i × DFU_i) ; main ≥ f(Σ DFU) Where:
-
DFU_i= Unit value assigned to fixture type i [–]
Worked example — A small apartment block with six WCs and six basins
The figures below were produced by running this page's own engine with the example inputs (Water closets (WCs), Lavatories (basins), Kitchen sinks, Showers, Bathtubs, Laundry trays, Dishwashers, Floor drains, Urinals, Code basis). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| 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. |
- Each WC contributes 3 units and each basin 1 unit under the IPC, with showers and baths at 2 each.
- Adding the kitchen, laundry, dishwashers, floor drains and urinals gives the total shown above.
- Look that total up in Table 710.1(1) for the minimum building drain.
Assumptions & limitations
Engineering calculation. Totals drainage fixture units and selects a building drain/sanitary main from the IPC sizing table. Local codes differ — check the table in force in your jurisdiction.
- Fixture unit values differ between IPC and UPC and between national amendments of each.
- Only the building drain is sized — branches, stacks, vents and trap arms each need their own table.
- No allowance for an indirect waste or a garbage disposal unit; add them from the code table.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IPC 2018 Table 706.1 — drainage fixture unit values.
- IPC 2018 Table 710.1(1) — sizing of horizontal drains and building drains.
- UPC 2021 Table 702.1 and Table 703.1 — fixture units and drain sizing.