Water supply pipe size for 80 fixture units over a 250 ft run

Notes

• 80 fixture units is 31 GPM, not 80 GPM, and that gap is the entire reason fixture units exist. Nobody runs every tap at once. The Hunter curve converts a count of fixtures into the flow that actually happens together, and it flattens hard: doubling the fixtures from 25 to 50 raises demand from 16 to 24 GPM, not from 25 to 50.
• Adding up the fixtures' rated flows instead is how houses end up with pipe two sizes too big. Oversized supply is not harmless: hot water takes longer to arrive because there is more cold water sitting in the line ahead of it, more water is wasted waiting, and the extra volume sits still long enough to matter for water quality.
• Velocity is the ceiling, and it is about noise and erosion, not about pressure. 1¼″ at 31 GPM runs 7.9 ft/s. Past 8 ft/s a copper line becomes audible and starts eroding the inside of its elbows; hot water is normally held to 5 ft/s because erosion accelerates with temperature.
• 25 psi at the fixture is marginal. Most fittings want 20 psi to work and 45 to feel good, and this calculation already assumed a healthy 60 psi at the meter. If the street pressure is lower than that, or the run is longer, the answer is a larger supply — or a booster pump if the street itself is the limit.
• The service line from the street is a separate question, and it is usually the real bottleneck. A 1¼″ building supply behind a ¾″ service from the main is limited by the ¾″. Older houses often have a galvanised service that has narrowed to half its bore with corrosion — no amount of repiping inside the house fixes that.
• Fixture unit values are not intuitive, so use the code table. A bathtub is 4 WSFU, a lavatory 1, a flush-tank toilet 2.2, a hose bib 2.5, a dishwasher 1.4. A typical three-bathroom house lands somewhere between 30 and 45 WSFU once the kitchen, laundry and outside taps are counted.