TryBuildCalcApartment Building Pipe Sizing Calculator (Multi-Family Riser Sizing)
Size a shared riser or main serving multiple apartment units.
🕒 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: 1-1/4" Copper (Type L)
Total load: 49.2 WSFU → estimated demand 28.68 GPM.
Fixture Unit Load
Lavatory (x6): 2.4 WSFU
Kitchen Sink (x6): 4.8 WSFU
Water Closet (1.6 GPF, Gravity Tank) (x6): 15 WSFU
Bathtub / Combo Bath-Shower (x6): 15.6 WSFU
Clothes Washer (x6): 11.4 WSFU
Demand Estimate
Building/Occupancy Type: Multi-Family / Apartments
System Type: Predominantly Flush-Tank Fixtures
Total Fixture Units (WSFU): 49.2
Fixture-Unit Demand: 28.68 GPM
Total Estimated Demand: 28.68 GPM
1.81 L/s, 6.51 m³/hr
Pipe Sizing
Pipe Material: Copper (Type L)
Maximum Velocity: 8 ft/s
Required Minimum Internal Diameter: 1.21 in (30.7 mm)
Recommended Nominal Pipe Size: 1-1/4"
Actual ID 1.265 in (32.1 mm) — velocity at this size: 7.32 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.
Multi-family riser sizing estimate
This page is set up for a shared riser or main serving 6 identical apartment units (each with 1 lavatory, 1 kitchen sink, 1 water closet, 1 bathtub, and 1 clothes washer) using the Multi-Family/Apartments occupancy column, which applies lower per-fixture WSFU values than a single-family home to reflect usage diversity across units. That gives 49.2 total WSFU (0.4 + 0.8 + 2.5 + 2.6 + 1.9, each x6).
On the flush-tank demand curve, 49.2 WSFU interpolates to an estimated 28.68 GPM. At 8 ft/s in Copper Type L, that needs a 1.21 in minimum diameter, rounding up to 1-1/4" (actual ID 1.265 in, actual velocity 7.32 ft/s).
- 6 apartment units x 5 fixtures each → 49.2 WSFU (multi-family column).
- Demand: 28.68 GPM.
- Recommended: 1-1/4" Copper Type L (actual velocity 7.32 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 x6, Kitchen Sink x6, Water Closet (1.6 GPF, Gravity Tank) x6, Bathtub / Combo Bath-Shower x6, Clothes Washer x6 |
| Building Type / System Type | multiFamily, flushTank |
| Pipe Material / Max Velocity | Copper (Type L), 8 ft/s |
Step 1 — Fixture Unit Load
| Fixture | Substitution | Result |
|---|---|---|
| Lavatory | 0.4 × 6 | 2.4 WSFU |
| Kitchen Sink | 0.8 × 6 | 4.8 WSFU |
| Water Closet (1.6 GPF, Gravity Tank) | 2.5 × 6 | 15 WSFU |
| Bathtub / Combo Bath-Shower | 2.6 × 6 | 15.6 WSFU |
| Clothes Washer | 1.9 × 6 | 11.4 WSFU |
| Total WSFU | 49.2 | |
Step 2 — Demand Flow Rate
| Calculation | Substitution | Result |
|---|---|---|
| Fixture-Unit Demand | Table lookup at 49.2 WSFU (flush-tank curve) | 28.68 GPM |
Step 4 — Required & Recommended Pipe Size
| Calculation | Substitution | Result |
|---|---|---|
| Required minimum ID | √(0.4085 × 28.68 ÷ 8) | 1.21 in |
| Recommended nominal size | Smallest Copper (Type L) size ≥ 1.21 in | 1-1/4" (actual velocity 7.32 ft/s) |
Therefore, this 49.2 WSFU load needs approximately 28.68 GPM total, sized to 1-1/4" Copper (Type L).
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.