Add up water supply fixture units (WSFU) and drainage fixture units (DFU) for any group of fixtures. The calculator converts WSFU to peak demand in gpm and L/s with Hunter's curve and sizes the horizontal branch, stack and building drain to the IPC.
Fixtures are never all used at once. Hunter's method gives each fixture a fixture unit value that reflects how much water it draws, for how long and how often, and then converts the total to a realistic peak flow. There are two separate systems: WSFU for sizing water supply pipes and DFU for sizing drains. A public flushometer water closet, for example, is 10 WSFU but only 4 DFU.
| Load (WSFU) | Flush tank systems | Flushometer valve systems |
|---|---|---|
| 5 | 9.4 gpm (0.59 L/s) | 15 gpm (0.95 L/s) |
| 10 | 14.6 gpm (0.92 L/s) | 27 gpm (1.70 L/s) |
| 20 | 19.6 gpm (1.24 L/s) | 35 gpm (2.21 L/s) |
| 50 | 29.1 gpm (1.84 L/s) | 50 gpm (3.15 L/s) |
| 100 | 43.5 gpm (2.74 L/s) | 67.5 gpm (4.26 L/s) |
| 200 | 65 gpm (4.10 L/s) | 90 gpm (5.68 L/s) |
| 500 | 124 gpm (7.82 L/s) | 143 gpm (9.02 L/s) |
| 1,000 | 208 gpm (13.12 L/s) | 208 gpm (13.12 L/s) |
From IPC Appendix E Table E103.3(3). Flushometer valves draw about 25 gpm for a few seconds, so small systems with flush valves need a much higher design flow; the two curves meet at about 1,000 WSFU.
| Fixture | WSFU private / public | DFU private / public |
|---|---|---|
| Water closet, flush tank (1.6 gpf) | 2.2 / 5.0 | 3 / 4 |
| Water closet, flushometer valve | 6.0 / 10.0 | 3 / 4 |
| Water closet, flushometer tank | 2.0 / 2.0 | 4 / 4 |
| Lavatory | 0.7 / 2.0 | 1 / 1 |
| Shower | 1.4 / 4.0 | 2 / 2 |
| Bathtub | 1.4 / 4.0 | 2 / 2 |
| Kitchen sink | 1.4 / 4.0 | 2 / 2 |
| Urinal, ¾" flushometer valve (1 gpf or less) | — / 5.0 | — / 2 |
| Bathroom group, flush tank | 3.6 / — | 5 / — |
| Size | Horizontal branch | Stack, ≤ 3 branch intervals | Building drain ¼ in/ft |
|---|---|---|---|
| 1½" | 3 | 4 | 1 |
| 2" | 6 | 10 | 21 |
| 2½" | 12 | 20 | 24 |
| 3" | 20 | 48 | 42 |
| 4" | 160 | 240 | 216 |
| 5" | 360 | 540 | 480 |
| 6" | 620 | 960 | 840 |
| 8" | 1,400 | 2,200 | 1,920 |
IPC Tables 710.1(1) and 710.1(2). Any drain serving a water closet must be at least 3". Values change slightly between IPC editions; check the edition adopted locally.
Next, size the water pipes with the Water Supply Pipe Size Calculator, check falls with the Drainage Slope Calculator, and see the plumbing symbols used on drawings.
Add up the water supply fixture units, then read the demand from Hunter's curve (IPC Table E103.3(3)), using the flushometer-valve column if the system has flush valves. 100 WSFU is 43.5 gpm with flush tanks and 67.5 gpm with flushometer valves.
WSFU (water supply fixture units) size the water piping; DFU (drainage fixture units) size the drains. They come from different tables and are not interchangeable.
Under the IPC, a 4-inch horizontal fixture branch carries 160 DFU, a 4-inch stack with up to three branch intervals 240 DFU, and a 4-inch building drain at 1/4 in/ft 216 DFU.
A 1.6 gpf flush-tank water closet is 2.2 WSFU in private use and 5 WSFU in public use; a flushometer-valve water closet is 6 and 10 WSFU. For drainage a 1.6 gpf water closet is 3 DFU private and 4 DFU public.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
No comments yet. Be the first to add one.