Method, limits and disclaimer

What this calculator is, and isn't

• It is a calculator. Like a mortgage or unit-conversion calculator, it applies published, public formulas and tables (listed below) to the numbers you enter, and shows its work.
• It doesn't know your project. It has not seen your drawings, site, fixtures, water utility, pipe routing, local amendments or any other conditions in play. It only knows what you typed.
• Results are not design values for any particular project. They are estimates to check against, not a design, a specification, a code-compliance determination or professional advice.
• You verify every result. You are responsible for your inputs and for confirming every result with the drawings, the engineer of record where there is one, and the code adopted where the work is done.
• It is not an engineering service. We do not review, approve, seal or take responsibility for any design, and no engineer–client relationship is created.

Drawings govern

Use the result as a minimum-requirements check. If the drawings show a larger pipe, price the drawing. If they show a smaller one, raise it as an RFI — it may be a change order.

Method

• Demand: Hunter's probability method (NBS BMS 65, 1940), curve 1 for systems mostly on flush valves and curve 2 for mostly flush tanks (NBS BMS 79, 1941, Fig. 8), log-log interpolation between Hunter's own points.
• Fixture units: BMS 79 Table 6 weights. Waterless urinals, floor drains, floor sinks, hub drains and condensate receptors count 0 FU and are listed at the top of the result. Continuous demand (hose bibbs, irrigation, equipment) is added on top.
• Pipe size: ASTM B88 Type L or K copper inside diameters; velocity limits from the Copper Development Association Copper Tube Handbook (8 ft/s cold, 5 ft/s hot to 140°F, 3 ft/s above); friction by Hazen-Williams (C = 140 in service, 150 new).
• Pressure budget: street pressure − rise × 0.434 − minimum pressure at the fixture − meter loss = pressure left for friction (BMS 79). Meter loss uses Hunter's 1921 meter ratings unless you enter the utility's or manufacturer's value — which is better for a modern meter.

Method: drainage

• Drainage fixture units: NBS BMS 66, Plumbing Manual (1940), Table 803. A separate scale from the water side; the two are never added. Fixtures the table doesn't list (floor drains, clothes washers, dishwashers…) are entered by you with their rating and where it came from, as BMS 66 Par. 803 allows (the rating of a comparable fixture with the same trap size).
• Pipe sizes: each run is sized for everything draining through it. Horizontal branches use Table 807's branch column; stacks use Table 805 (or Table 805(b)-III when detailed plans are approved); the building drain uses Table 807 at the chosen fall (¼"/ft standard). Fixture units govern. The WC count rule (default: up to 3 WCs on 3", the 4th → 4") can only make a pipe larger, and no pipe is smaller than a run feeding it.
• Vents through roof: default check is that the VTR diameters add up to at least the building drain diameter (4" drain → 2 × 2" or 3" + 2"). Venting rules vary a lot by state, city and climate: some codes use vent area, cold-climate codes enlarge vents through the roof. Use your local code's method and minimums.

Limits

• Range: Hunter's curves run from about 6 FU (flush tanks) or 10 FU (flush valves) to 2,990 FU. Outside that range, and for demand beyond 8-inch copper, the calculator stops and says so instead of guessing.
• Low-flow fixtures: Hunter's data are from 1940-era fixtures. Modern low-flow fixtures use less water, so results tend to be larger than actual demand (the safe side). Sizes are not reduced for this. For dwellings, UPC Appendix M (Water Demand Calculator) gives a lower estimate where the local code allows it.
• Hot and cold: choose what the pipe carries. A building supply or main carries both (total fixture units); a hot-only or cold-only line uses ¾ of the total (Hunter, BMS 79 Table 6 note 3). Continuous demand is added as entered.
• Copper only. No PEX, CPVC or other materials yet.
• Not included: fire sprinklers, storm drainage, detailed vent sizing (individual, wet, circuit vents, trap arms), pipe offsets, gas, medical gas, process piping, water heater sizing, backflow devices, or local amendments.
• Codes differ. Your jurisdiction may require a different method (for example IPC Appendix E or UPC Appendix A/M). Use what your code official accepts.

Every result is stamped

Each saved result carries the calculator version, the date and time (UTC), the source list and this disclaimer, so you have a record of what was calculated and how.

Sources

• R. B. Hunter, Methods of Estimating Loads in Plumbing Systems, NBS BMS 65 (1940)
• R. B. Hunter, Water-Distributing Systems for Buildings, NBS BMS 79 (1941)
• Central Housing Committee, Plumbing Manual, NBS BMS 66 (1940): drainage fixture units and drain sizes
• ASTM B88, Standard Specification for Seamless Copper Water Tube (dimensions)
• Copper Development Association, Copper Tube Handbook, Sec. III
• Hazen-Williams C-factor tables; fixture manufacturers' published minimum pressures (e.g. Flushmate)

Full terms: Terms of Service.