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Start with fixture units (IPC Table E103.3(2) WSFU / Table 709.1 DFU), then storage water heater, expansion tank, drain slope, DWV vent, water-supply pipe, septic tank, basement sump pump, well pump, salt-based water softener, and fuel-gas pipe sizing (IFGC Table 402.4 / NFPA 54). Starting point, not a plumber or an inspector.
Nine plumbing jobs. Start by totaling WSFU and DFU, then size the heater, the expansion tank on a closed main, the drain fall, the vent, the supply pipe, the septic tank if you have one, the basement sump, and a salt-based water softener when hardness is the problem. Water heating is first-hour rating, not the number on the carton. A closed supply (PRV or backflow preventer) needs an expansion tank sized from heater gallons and fill pressure, not from FHR. Horizontal drains are Table 704.1 slope. Vents are DFU and developed length (IPC 906.2). Incoming water pipe is WSFU, length, and pressure (IPC Table E104.1) — not the same fixture units as the drain. A sump is groundwater GPM at a head, not fixture units. None of these replace a plumber walking the house or the code on the permit card.
Start here · Fixture units
Total WSFU from IPC 2021 Table E103.3(2) and DFU from Table 709.1 from a real fixture list. The on-ramp into supply-pipe, vent, and slope sizing — two separate totals, two separate calculators.
Open Fixture Unit Calculator ->Water heating · Sizing
Recommended storage tank size (gallons) and first-hour rating from household size, bathrooms, peak-usage scenario, and gas vs electric recovery. DOE Energy Saver peak-hour worksheet. Starting point — not a plumber’s fixture survey.
Open Water Heater Sizing Calculator ->Water heating · Expansion
Minimum potable expansion-tank gallons from heater volume, incoming pressure, PRV setting, and temperature rise. Boyle’s-law acceptance at 150 psig T&P. Starting point — not a hydronic boiler tank or a permit.
Open Expansion Tank Sizing Calculator ->DWV · Slope
Minimum total drop over a horizontal drain run from IPC 2021 Table 704.1 by pipe diameter and length, plus inches-per-foot, percent, UPC 708.1 comparison, and a joist-bay headroom flag. Prescriptive starting point — not a substitute for local code.
Open Drain Pipe Slope Calculator ->DWV · Vent sizing
Minimum individual/branch vent diameter from IPC 2021 Section 906.2 by DFU load and developed length, plus UPC Table 703.2 comparison. Individual/branch only — not a wet vent, stack vent, or local code.
Open DWV Vent Sizing Calculator ->Supply · Pipe sizing
Minimum trunk/branch diameter from IPC 2021 Table E104.1 by water-supply fixture units, developed length, and pressure, plus UPC Table 610.4 comparison. Fixture-unit method only — not a hydraulic calc, not DFU, not a permit.
Open Water Supply Pipe Sizing Calculator ->Onsite · Tank sizing
Planning minimum tank gallons from IPSDC Table 802.7.1 by bedroom count, with local adders. Bedroom tables diverge by state — call the health department before you buy a tank.
Open Septic Tank Sizing Calculator ->Basement · Dewatering
Required pump GPM, 1/3 / 1/2 / 3/4 HP class, and estimated cycle time from basin diameter, groundwater inflow, and total discharge head. Starting point — not a named-model pump curve.
Open Sump Pump Sizing Calculator ->Well · Potable Supply
TDH from well depth, fixture lift, pipe friction, and cut-out PSI; recommended 1/2–1.5 HP class and pressure-tank gallons. Starting point — not a named-model pump curve. Not a sump pump.
Open Well Pump Sizing Calculator ->Treatment · Grain capacity
Salt-based ion-exchange grain class from household size, hardness (GPG or ppm), and optional iron. 7-day cycle with a 25% reserve. Starting point — not a water test, not a salt-free conditioner.
Open Water Softener Sizing Calculator ->Fuel gas · Pipe sizing
Minimum Schedule 40 or CSST size from IFGC Table 402.4 by BTU load, developed length, and fittings — natural gas or propane. Starting point — verify local fuel gas code.
Open Gas Pipe Sizing Calculator ->No. Water-heater numbers use the public DOE peak-hour worksheet. Expansion-tank numbers use water density and Boyle’s-law acceptance at 150 psig T&P — not a hydronic boiler tank. Drain-slope numbers use IPC Table 704.1 and compare UPC 708.1. Vent numbers use IPC 906.2 / UPC Table 703.2 for individual/branch vents only. Water-supply pipe numbers use IPC Table E104.1 / UPC Table 610.4 by WSFU — not DFU. Sump numbers use 1.5× measured inflow and typical residential HP bands, not a named pump curve. Unusual patterns, grease lines, 8-inch sewers, long runs with a PRV, artesian pits, and local amendments need a plumber and the adopted code book.
Water heating, DWV, septic, and basement sumps are plumbing. HVAC hubs stay on duct, static pressure, superheat, and delta-T.
No. A sump is groundwater (or weeping tile) pumped to a legal outlet. Gravity house drains are Table 704.1 slope. Vents are a different DWV chapter. Do not dump a sump into the septic tank.
No. A well pump delivers potable supply under continuous pressure (TDH, HP class, pressure tank). A sump moves basement groundwater. Use the well pump sizing calculator for wells and the sump tool for crocks.
No. IPC Table 704.1 is three rows. The calculator already maps diameter to slope and drop. A standalone reprint of the code table would not add a reference the calculator and guide do not already give you.
No. Gallons per inch is a one-line cylinder formula, and GPM at head is already the calculator result. A 2D reprint would not add a reference the calculator and guide do not already give you.
No. Softeners are grains of hardness between regenerations (people × gallons × GPG, 7-day cycle, 25% reserve). Well pumps are GPM at total dynamic head. Water heaters are first-hour rating. Use the water softener sizing calculator for salt-based ion exchange only — salt-free conditioners are a flow-rate product, not a grain class.
No. Required tank size is a 2D function of heater volume, fill pressure, and temperature rise. A 1D “50 gal heater = 2 gal tank” table would be wrong at 80 psi fill, and a 2D reprint is what the calculator already produces for any combination you type.
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