TryBuildCalcMain Water Line Sizing Calculator (Building Main Service Line Sizing)
Size a building's main water service line from its combined fixture-unit load.
🕒 Last updated: July 31, 2026
Building & System Type
ℹ️Sets which WSFU value applies to each fixture — the same fixture carries a different load in a single home versus an apartment building versus a commercial building.
ℹ️Flushometer-valve systems demand much more flow at the same fixture-unit count than flush-tank systems — select whichever matches most of the water closets/urinals on this pipe segment.
Fixtures
Quick-fill a starting point, then add/remove/edit rows as needed:
Fixture 1
Fixture 2
Fixture 3
Fixture 4
Fixture 5
Only fixtures listed for the selected Building/Occupancy Type are shown — up to 30 fixture rows total.
Continuous-Flow Fixtures (Optional)
Hose bibbs and lawn sprinkler zones are continuous-use, not intermittent — per IPC Appendix E they're added directly in GPM here, not as fixture units above.
Pipe Sizing
ℹ️Sets which standard nominal sizes and internal diameters the recommended size is rounded up to.
ℹ️Defaults to 8 ft/s for cold water branches — commonly lowered to around 5 ft/s for hot water piping.
🔧 Recommended pipe size: 2" Galvanized Steel (Schedule 40)
Total load: 81.8 WSFU → estimated demand 62.54 GPM.
Fixture Unit Load
Lavatory (x10): 8 WSFU
Kitchen Sink (x8): 8.8 WSFU
Water Closet (1.6 GPF, Flushometer Valve) (x8): 40 WSFU
Urinal (1.0 GPF or less) (x4): 16 WSFU
Shower (x6): 9 WSFU
Demand Estimate
Building/Occupancy Type: Other Than Dwelling (Commercial/Public)
System Type: Predominantly Flushometer-Valve Fixtures
Total Fixture Units (WSFU): 81.8
Fixture-Unit Demand: 62.54 GPM
Total Estimated Demand: 62.54 GPM
3.95 L/s, 14.2 m³/hr
Pipe Sizing
Pipe Material: Galvanized Steel (Schedule 40)
Maximum Velocity: 8 ft/s
Required Minimum Internal Diameter: 1.787 in (45.4 mm)
Recommended Nominal Pipe Size: 2"
Actual ID 2.067 in (52.5 mm) — velocity at this size: 5.98 ft/s
Design Notes
This is a velocity-limited quick-sizing estimate, not a full hydraulic pressure-loss design. For anything beyond routine residential/light-commercial branch sizing, confirm with a full IPC Appendix E / UPC Table 610.3 pressure-loss calculation accounting for developed length, fittings, elevation change, and available pressure.
Hose bibbs and sprinkler zones are continuous-flow demand, not fixture units — per IPC Appendix E they are excluded from the fixture-unit/demand-curve method and added directly in GPM on top of it.
Approximate results for planning only. Verify with a professional.
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Pipe Sizing by Fixture Units Calculator.
Building main service line sizing estimate
This page is set up for a larger commercial/institutional building's main service line — 10 lavatories, 8 kitchen sinks, 8 flushometer-valve water closets, 4 urinals, and 6 showers combined from across the building — giving 81.8 total WSFU on the Other Than Dwelling occupancy column.
Since the water closets are flushometer-valve type, the flushometer-valve demand curve applies, giving an estimated 62.54 GPM. At 8 ft/s in Galvanized Steel Schedule 40, that needs a 1.787 in minimum diameter, rounding up to 2" (actual ID 2.067 in, actual velocity 5.98 ft/s).
- 36 fixtures combined across the building → 81.8 WSFU.
- Flushometer-valve demand: 62.54 GPM.
- Recommended: 2" Galvanized Steel Schedule 40 (actual velocity 5.98 ft/s).
How Pipe Sizing by Fixture Units Works
The calculation happens in four stages — fixture-unit load, demand flow rate, an optional continuous-flow addition, then pipe diameter.
Step 1 — Total Fixture Unit Load
Total WSFU = Sum of every fixture row's WSFU
Each fixture's WSFU value depends on the selected Building/Occupancy Type — the same fixture is rated lower in a multi-family building than a single-family home, reflecting greater usage diversity across more dwelling units sharing the same riser.
Step 2 — Demand Flow Rate (GPM)
This relationship is deliberately nonlinear — doubling the fixture-unit total does not double the demand flow rate, since the probability of every fixture running simultaneously keeps dropping as the count grows. Flushometer-valve systems draw a much higher flow at the same WSFU total than flush-tank systems, since a flush valve delivers its full flush volume quickly with no tank to buffer the draw.
Step 3 — Add Continuous-Flow Demand (Optional)
Hose bibbs, lawn sprinkler zones, and similar continuous-use connections are excluded from the fixture-unit method entirely — per IPC Appendix E, fixture units and the demand curve model intermittent, probability-of-simultaneous-use demand, which doesn't apply to a fixture that can run continuously for an extended period. Their GPM is instead added directly on top of the fixture-unit-derived demand (IPC's own worked example: 2 hose bibbs × 5 GPM = 10 GPM, added to the WSFU-derived total).
Step 4 — Required Pipe Diameter
Recommended Nominal Size = smallest standard size for the chosen material whose actual ID ≥ Required Minimum ID
0.4085 comes directly from converting GPM to cubic feet per second and expressing pipe area from a diameter in inches — this is a standard plumbing/HVAC flow-velocity relationship. Since pipe only comes in discrete standard sizes, the required diameter is always rounded up, never down, and the actual velocity at the recommended size (shown in the result) will sit at or below your chosen maximum.
Worked Example
This example uses the active inputs above and follows the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| Fixtures | Lavatory x10, Kitchen Sink x8, Water Closet (1.6 GPF, Flushometer Valve) x8, Urinal (1.0 GPF or less) x4, Shower x6 |
| Building Type / System Type | otherThanDwelling, flushValve |
| Pipe Material / Max Velocity | Galvanized Steel (Schedule 40), 8 ft/s |
Step 1 — Fixture Unit Load
| Fixture | Substitution | Result |
|---|---|---|
| Lavatory | 0.8 × 10 | 8 WSFU |
| Kitchen Sink | 1.1 × 8 | 8.8 WSFU |
| Water Closet (1.6 GPF, Flushometer Valve) | 5 × 8 | 40 WSFU |
| Urinal (1.0 GPF or less) | 4 × 4 | 16 WSFU |
| Shower | 1.5 × 6 | 9 WSFU |
| Total WSFU | 81.8 | |
Step 2 — Demand Flow Rate
| Calculation | Substitution | Result |
|---|---|---|
| Fixture-Unit Demand | Table lookup at 81.8 WSFU (flush-valve curve) | 62.54 GPM |
Step 4 — Required & Recommended Pipe Size
| Calculation | Substitution | Result |
|---|---|---|
| Required minimum ID | √(0.4085 × 62.54 ÷ 8) | 1.787 in |
| Recommended nominal size | Smallest Galvanized Steel (Schedule 40) size ≥ 1.787 in | 2" (actual velocity 5.98 ft/s) |
Therefore, this 81.8 WSFU load needs approximately 62.54 GPM total, sized to 2" Galvanized Steel (Schedule 40).
This does not include developed pipe length, fittings, elevation, or available service pressure. Cross-check against the Reference Tables in the complete Pipe Sizing by Fixture Units Calculator.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Pipe Sizing by Fixture Units Calculator.