Pipe Sizing
Water supply by fixture units; drainage by DFU.
Example
You enter
- Fixtures fixture lavatory, count 2, and 3 more
- Drain slope 0.25
You get
- Total WSFU 10.5
- Total DFU 12
- Estimated demand 14.8 gpm
- Recommended supply size 1
- Recommended drainage size 3
Checked against published worked examples from: IPC / UPC fixture-unit tables (project bundled).
Details, formula, and sources
Formula or tableWSFU from UPC-style private-use fixture-unit values (above IPC Appendix E Table E103.3(2)); Hunter's Curve converts total WSFU to gpm; DFU per IPC 2027 Table 709.1 (unchanged from 2021 except the drinking fountain, now 0); supply size from the bundled gpm thresholds; drain size is the smallest IPC 2027 Table 710.1(1) building drain that carries the DFU at the chosen slope, and 3 in minimum with a water closet (Table 709.1 outlet, 704.2).
Source and editionIPC 2027 (International Plumbing Code). Tables 709.1 and 710.1(1) and Appendix E by name; Hunter's Curve (NBS BMS65).
Free accessFree read-only access at codes.iccsafe.org.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namesfixtures, slope_in_per_ft, total_wsfu, total_dfu, estimated_demand_gpm, recommended_supply_size, recommended_drainage_size
- Pressure head at most-distant fixture not checked here (IPC Table 604.3 sets the flow pressure at each outlet: 8 psi for most fixtures, 20 psi for thermostatic or pressure-balancing mixing valves, more for flush valves)IPC 2027 Table 604.3 (cited as 604.6, variable street pressures, until 2026-10-17)
- Maximum velocity not checked here (8 fps water supply / 5 fps hot water is common practice)engineering practice, not an IPC rule (604.4, cited here until 2026-10-17, limits fixture flow rates)