Water supply pipe size for 50 fixture units over a 100 ft run
| Building supply sizeType L copper, 1.265″ bore | 1¼″ |
|---|---|
| Simultaneous demandfrom the Hunter curve — the whole point of fixture units | 24GPM |
| Supply fixture units | 50WSFU |
| Velocity at that flowceiling is 8 ft/s for cold, 5 for hot | 6.1ft/s |
| Friction over the run100 ft of 1¼″ at 24 GPM | 5.5psi |
| Pressure left at the fixtureassuming 60 psi at the main, an 8 psi meter, and one storey of rise | 42psi |
Notes
- 50 fixture units is 24 GPM, not 50 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 24 GPM runs 6.1 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.
- 42 psi left at the fixture is comfortable. The assumptions behind that are worth checking against your own house: 60 psi at the meter, 8 psi lost through the meter itself, and one storey of rise at 0.43 psi per foot.
- 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.
50 fixture units is 24 GPM
Not 50 GPM, and that gap is the entire reason fixture units exist. Nobody opens every tap at once. The Hunter curve converts a count of fixtures into the flow that realistically happens together, and it flattens hard as the count grows.
Over a 100 ft developed length that demand calls for 1¼″ pipe, running at 6.1 ft/s and losing 5.5 psi to friction along the way.
Bigger is not safer
Adding up each fixture's rated flow instead of using the curve is how houses end up with supply pipe two sizes too large. Oversized pipe holds more cold water ahead of the hot, so hot water takes longer to arrive and more is wasted waiting for it. The water also sits still longer, which matters for quality.
The service line usually decides everything
1¼″ inside the house behind a ¾″ service from the street is limited by the ¾″. On older properties the service is often galvanised and has corroded to half its original bore — no amount of repiping inside the walls fixes that, and it is the first thing to check when pressure is poor everywhere at once.
Counting fixture units
A bathtub is 4, a lavatory 1, a flush-tank toilet 2.2, a hose bib 2.5, a dishwasher 1.4. A typical three-bathroom house lands between 30 and 45 once the kitchen, laundry and outside taps are counted.
Nearby sizes
- Water supply pipe size for 40 fixture units over a 100 ft run
- Water supply pipe size for 60 fixture units over a 100 ft run
- Water supply pipe size for 30 fixture units over a 100 ft run
- Water supply pipe size for 80 fixture units over a 100 ft run
- Water supply pipe size for 50 fixture units over a 50 ft run
- Water supply pipe size for 50 fixture units over a 150 ft run
- Water supply pipe size for 50 fixture units over a 200 ft run
Read more
- Low Water Pressure at One Faucet Only — Five Causes, Cheapest FirstOne weak tap is never a pressure problem. Pressure is shared by the whole house — this is a restriction, and it is usually within a foot of the spout.
- Whole-House Repipe Cost by State — 12 Fixtures, All 51Repipe is 70% labour, so the spread between states is wider here than on almost any other trade job. The table shows all 51 and where each number comes from.
- Connecting PEX to Copper — Four Methods and the One Real RiskMixing the two is normal and code-compliant. The failures come from heat, from the wrong fitting for the tubing, and from sunlight — not from the metals touching.
- Drain Gurgles When the Washing Machine Empties — A Venting ProblemThe gurgle is the sound of a trap seal being emptied. Get it wrong for long enough and the smell that follows has an obvious source.