Water Supply Fixture Units (WSFU) Calculator: Peak Demand and Pipe Size
Add up water supply fixture units with the IPC load values, convert WSFU to probable peak demand in gpm with the IPC Table E103.3(3) Hunter curve (flush-tank or flushometer systems), and pick the copper pipe size that keeps velocity under 8 ft/s (5 ft/s for hot water), with friction loss per 100 ft and a size table.
How it works
WSFU = Σ fixture count × load value (IPC Table E103.3(2), private: water closet tank 2.2, lavatory 0.7, shower or tub 1.4, kitchen sink 1.4, dishwasher 1.4, clothes washer 1.4, hose bibb 2.5 + 1.0 each additional) · Peak demand (gpm) = IPC Table E103.3(3) at that WSFU, interpolated · Velocity (ft/s) = 0.4085 × gpm ÷ ID² (in) · Pipe = smallest size with velocity ≤ limit · Friction (psi/100 ft) = 452 × gpm^1.852 ÷ (C^1.852 × ID^4.87), C = 140
Fixtures rarely all run at once, so plumbing codes size water pipes with water supply fixture units: weights that reflect each fixture’s flow, how long it runs and how often it is used. The International Plumbing Code lists them in Table E103.3(2); in a home a flush-tank toilet counts 2.2, a lavatory 0.7, and a shower, tub, kitchen sink, dishwasher or clothes washer 1.4 each. The first hose bibb counts 2.5 and each additional one 1.0. Roy Hunter’s probability method then turns the total into the flow that is rarely exceeded, tabulated in IPC Table E103.3(3). The curve is strongly non-linear: 20 WSFU in a flush-tank system is 19.6 gpm, but 200 WSFU is only 65 gpm. Buildings with flushometer-valve toilets use a higher column, because each flush draws a large flow for a few seconds. With the design flow known, the calculator picks the smallest copper size that keeps velocity under your limit, about 8 ft/s for cold water and 5 ft/s for hot, to avoid noise, erosion and water hammer. It also shows the friction loss per 100 ft from the Hazen-Williams formula, which is what governs when pressure is low or runs are long. The separate hot and cold mains each carry three quarters of the total load. Modern low-flow fixtures make Hunter’s numbers conservative for homes, which is why the IAPMO Water Demand Calculator is now an option for residential work; local code and a licensed designer have the final say.
Worked example
A 3-bath house with 3 flush-tank toilets (6.6 WSFU), 3 lavatories (2.1), 2 showers (2.8), a kitchen sink, dishwasher and clothes washer (4.2) and 2 hose bibbs (2.5 + 1.0 = 3.5) totals 19.2 WSFU. The IPC table gives 19.2 gpm at 19 WSFU and 19.6 gpm at 20, so the probable peak is about 19.3 gpm. In 1″ copper type L (ID 1.025″) that flows at 0.4085 × 19.28 ÷ 1.025² = 7.5 ft/s, within the 8 ft/s limit, while 3/4″ (ID 0.785″) would run at 12.8 ft/s. The friction loss in 1″ is about 10.2 psi per 100 ft (Hazen-Williams, C = 140), and each of the hot and cold mains alone (14.4 WSFU) carries about 17.2 gpm.
Sources: City of Naples: Table for estimating demand (IPC Table E103.3(3) values) · Open Exam Prep: WSFU and demand estimation (IPC Table E103.3(2) loads, worked examples) · ICC: 2021 International Plumbing Code, Appendix E
Frequently asked questions
How many fixture units is a bathroom?
A private bathroom group with a flush-tank toilet, lavatory and tub or shower counts 3.6 WSFU in the IPC (2.7 cold, 1.5 hot). Counted fixture by fixture it is 2.2 + 0.7 + 1.4 = 4.3, so the group value gives a slightly smaller total for the same room.
How do I convert WSFU to gpm?
Look the total up in IPC Table E103.3(3) and interpolate between rows. For flush-tank systems 10 WSFU is 14.6 gpm, 20 is 19.6, 50 is 29.1 and 100 is 43.5 gpm. Flushometer-valve systems use a higher column: 50 WSFU is 50 gpm.
Why are hose bibbs weighted so heavily?
Because a hose runs at a high, steady flow for a long time. The IPC counts the first hose bibb at 2.5 WSFU and each additional one at 1.0. An irrigation system should be added as its actual zone flow in gpm instead.
What is the maximum water velocity in copper pipe?
The copper industry recommends about 8 ft/s (2.4 m/s) for cold water and 5 ft/s (1.5 m/s) for hot water, because hot water erodes copper faster. Many designers use 4–6 ft/s in quiet buildings.
Flush tank vs flushometer: which column should I use?
Use flush tanks for homes and most small buildings. Use flushometer valves when the toilets or urinals are tankless commercial valves, which draw 25 gpm or more for a few seconds; the demand for the same WSFU is then higher up to about 1,000 WSFU, where the two columns meet.
Is velocity sizing enough?
Only when pressure is ample. The full IPC Appendix E method subtracts elevation (0.433 psi per foot), meter loss and the minimum pressure the highest fixture needs from the street pressure, then sizes pipe so friction over the developed length fits in what is left.
Related calculators
Simplified sizing using the IPC Appendix E tables and standard pipe-flow formulas. Local codes, listed product data and a licensed plumber or engineer govern the final design; verify pressure, temperature and fixture requirements on site.