When to use: Use to size water supply branches and mains from total fixture unit load per IPC Section 604 and Table 604.3. Enter the total water supply fixture units (WSFU) served by the pipe segment. The calculator applies Hunter's curve to estimate peak demand flow (GPM) and selects the minimum IPC-compliant pipe size. Use for domestic cold and hot water systems, branches, risers, and building mains.
This calculator sizes water supply pipes from total water supply fixture units (WSFU) using IPC Table 604.3 and Hunter's curve, determining peak demand flow and the minimum compliant pipe diameter. Plumbing engineers use it to design domestic cold and hot water supply mains, branches, and risers in compliance with the International Plumbing Code.
IPC Section 604 requires that water supply piping be sized to provide adequate flow at minimum pressure (15 psi at ordinary fixtures, 25 psi at flush valve closets) under peak simultaneous demand. The process starts with fixture unit tallies per Table 604.1 (computed with the Fixture Unit Calculator). Hunter's curve then converts total WSFU to peak demand GPM — an empirical probability-based relationship developed by Roy B. Hunter that accounts for the statistical reality that not all fixtures operate simultaneously.
With peak GPM known, IPC Table 604.3 gives the minimum pipe size that satisfies pressure drop limits for the pipe run length and available pressure. The Hazen-Williams equation V = 1.318 × C × R^0.63 × S^0.54 (where R is hydraulic radius, S is slope, and C is the Hazen-Williams roughness coefficient) underpins the table values. Supply velocity limits from IPC are implicitly enforced through the table — 5–8 ft/s for domestic water and 10 ft/s for fire protection.
The International Plumbing Code (IPC) Section 604 governs water supply pipe sizing in states that have adopted it. The Uniform Plumbing Code (UPC) Chapter 6 provides equivalent requirements with some differences in fixture unit values and pipe capacity tables. ASME A112.18.1 and CSA B125.1 govern fittings and valves. Minimum pipe sizes for individual fixtures are set by IPC Section 604.5. For fire suppression branch connections from domestic water, NFPA 13 and local ordinances may require separate service entrances or backflow preventers.
The velocity limits embedded in IPC Table 604.3 prevent two failure modes: excessive pressure drop and erosion-corrosion. Velocities above 8 fps in copper supply piping cause flow-accelerated corrosion at fittings, particularly in soft water and low-pH conditions. At the other extreme, velocities below 0.5 fps allow sediment accumulation and biological growth (Legionella risk) in stagnant sections. For tall buildings, the pressure available at each floor must be verified — a 100 ft rise consumes 43.3 psi of static pressure at 0.433 psi/ft, which may require pressure-reducing valves (PRVs) at lower floors if street pressure is high.
Enter the total WSFU for the pipe segment being sized — this is the sum of fixture units for all fixtures served by this section. For a building main, enter the total building WSFU; for a branch, enter only the fixtures on that branch. Select the system type (cold, hot, or combined). The calculator applies Hunter's curve to convert WSFU to peak GPM and looks up the minimum IPC-compliant pipe size. Next, verify the result against available pressure: subtract static head loss (0.433 psi per foot of rise) and friction losses from the available street or booster pressure to confirm minimum residual pressure at the highest fixture.
IPC does not state an explicit velocity limit but the capacity tables are designed around 5–8 fps for supply branches and mains. ASHRAE and ASPE recommend a maximum of 8 fps for cold water and 5 fps for hot water above 140°F to prevent flow-accelerated corrosion in copper. At minimum, velocity should exceed 0.5 fps to prevent sediment accumulation and biological fouling.
When calculated residual pressure at the highest fixture falls below the IPC minimum (15 psi standard, 25 psi flush valve), a booster pump system is required. Booster pump stations typically include duplex or triplex pumps with variable frequency drives, a bladder-type surge tank, and pressure sensors for automatic staging. The required boost pressure equals the deficit between available residual and minimum required pressure, plus a control margin of 5–10 psi.
Hot water supply pipe sizing follows the same Hunter's curve and IPC tables, but with hot WSFU values from Table 604.1. In addition, hot water recirculation must be provided for long runs to prevent stagnation and Legionella risk — ASHRAE 188 requires water temperature above 122°F throughout the distribution system. Recirculation pipe sizing is based on the heat loss of the hot water mains, not on fixture demand.
Yes. PEX-A, PEX-B, and PEX-C are all IPC-compliant for domestic cold and hot water supply. PEX has a smooth interior (equivalent to Hazen-Williams C = 150) and is more resistant to freeze damage than copper due to its flexibility. However, PEX is not permitted in all jurisdictions for outdoor exposed installation and has different UV degradation considerations. Size PEX using the same IPC tables as copper — the capacity is essentially the same at equal inside diameters.
The building water service carries the total peak demand for the entire building, found from total building WSFU and Hunter's curve. The service must also deliver fire hose allowance when the domestic and fire systems share a single service entrance. Most jurisdictions require a separate fire service line above certain building heights or occupancies. The service pipe is sized so that pressure drop from the utility meter to the building main shutoff does not exceed 5–10 psi at peak flow.
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