Sources & formulas

Every calculator on PipeCalcs rests on established arithmetic and physics plus stable plumbing conventions. Unlike topics with a time dependency, "verification" here is mathematical: each formula is tested against known values and is therefore correct for good. Here are the technical foundations by area.

Physics & units

  • Volume: 1 cubic foot = 7.48 US gallons; pipe water volume gal = 0.0408 × diameter_in² × length_ft.
  • Pressure ↔ head: 1 psi = 2.31 ft of head.
  • Heat: BTU to warm water = gallons × 8.34 lb/gal × ΔT°F; 1 kWh = 3412 BTU.
  • Gas: CFH = BTU/hr ÷ heating value (natural gas ~1000, propane ~2516 BTU/cu ft).

Water heater

  • Tankless load: gas BTU/hr = GPM × 500 × temperature rise; electric kW = GPM × ΔT × 0.1465.
  • Recovery: GPH = energy BTU/hr × efficiency ÷ (8.34 × ΔT).
  • Tank sizing: match the first-hour rating to peak-hour demand (labeled planning band).

Supply, drain & vent sizing (labeled planning bands)

  • Peak demand: water-supply fixture units (WSFU) → GPM on a Hunter-curve band; supply size from the GPM capacity band.
  • Drain & vent: drainage fixture units (DFU) → minimum drain and vent size (IPC/UPC-style band); drain fall = length × slope (1/4 in per foot up to 2-1/2 in).
  • Gas pipe: capacity (CFH) by size and longest run (NFPA-54-style band); your local code and a licensed gas fitter govern.

Cost, savings & comparison

  • Every cost tool uses the prices you enter ($/fixture, $/ft, $/hour, $/gallon) — no material or service price, labor rate, cost index or live loan rate is stored, so the site needs no maintenance.
  • Cost per fixture: your total ÷ fixtures.
  • Savings: (old − new flow) × uses × days × your $/gal; comparisons are pure identities on your figures — illustrative only.

Permit rules, setbacks, local code and inspector requirements vary by place and change over time — always confirm sizing, materials and permits with a licensed plumber and your local building department, and remember that all gas work must be done by a licensed professional with a permit and inspection.