Plumbing unit conversions
These are the fixed physics conversions the calculators use — not prices, not conventions that drift. A cubic foot holds 7.48 US gallons; pressure and head relate as 1 psi = 2.31 ft; gas flow converts as BTU/hr ÷ heating value = CFH (nat gas 1000, propane 2516); and pipe volume is 0.0408 × diameter² × length. Apply them in the pipe volume, tankless sizing and BTU ↔ CFH tools.
| Quantity | Conversion | Used for |
|---|---|---|
| 1 cubic foot | 7.48 US gallons | Volume conversions |
| Pressure ↔ head | 1 psi = 2.31 ft | Water pressure & elevation |
| Pipe water volume | gal = 0.0408 × d_in² × L_ft | Time-to-hot, purge volume |
| Water weight | 1 gallon = 8.34 lb | Heat energy (BTU = gal × 8.34 × ΔT) |
| Natural gas | CFH = BTU/hr ÷ 1000 | Gas flow / pipe capacity |
| Propane / LP | CFH = BTU/hr ÷ 2516 | Gas flow / pipe capacity |
| Energy | 1 kWh = 3412 BTU | Electric water heating |
Fixed physics conversions, not prices or drifting conventions — the arithmetic backbone of the calculators.