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Plumbing

Water Pipe Size Calculator

Total water supply fixture units, read peak GPM from Hunter's curve, and size the service or main by velocity — in copper, PEX, or CPVC, with the math shown.

Fixtures
Bathroom group (WC + lav + tub/shower)
3.6 WSFU
Water closet (1.6/1.28 gpf)
2.2 WSFU
Lavatory (bathroom sink)
0.7 WSFU
Shower stall
1.4 WSFU
Bathtub (with shower)
1.4 WSFU
Kitchen sink (+ disposal / DW on same trap)
1.4 WSFU
Dishwasher (separate)
1.4 WSFU
Clothes washer standpipe
1.4 WSFU
Laundry tray / utility sink
1.4 WSFU
Bar / prep sink
1 WSFU
Hose bibb / sillcock
2.5 WSFU
Water closet type

Flushometer valves draw a higher peak demand for the same fixture units.

Inside diameter drives the velocity — plastic runs smaller than copper.

Advanced assumptions
fps

8 fps is the common cold-water limit for copper. Use ~5 fps on hot-water lines.

Service / main size
Copper Type L
3/4″copper
WSFU
7.5
Peak demand
7.3gpm
Pipe size
3/4″
Velocity
4.8fps
Fixture tally (3)
Bathroom group (WC + lav + tub/shower)1 × 3.63.6 WSFU
Kitchen sink (+ disposal / DW on same trap)1 × 1.41.4 WSFU
Hose bibb / sillcock1 × 2.52.5 WSFU
ƒShow the math
  1. 1.Total water supply fixture unitsper IPC 2021 Table E103.3(2)
    Σ qty × WSFU per fixture
    1×3.6 + 1×1.4 + 1×2.5
    = 7.5 WSFU
  2. 2.Peak demand (Hunter's curve)per IPC 2021 Table E103.3(3)

    Interpolated on the flush-tank demand curve.

    WSFU → GPM
    7.5 WSFU
    = 7.3 GPM
  3. 3.Smallest Copper Type L at ≤ 8 fps

    3/4″ inside diameter 0.785 in.

    = 3/4″
  4. 4.Velocity at that size
    0.4085 × GPM / ID²
    0.4085 × 7.3 / 0.785²
    = 4.8 fps
  5. 5.Estimated friction lossper Hazen–Williams, per 100 ft (informational)
    4.52 × Q^1.85 / (C^1.85 × d^4.87) × 100 ft
    4.52 × 7.3^1.85 / (150^1.85 × 0.785^4.87) × 100
    = 5.5 psi / 100 ft
Before you rely on this
  • PEX and CPVC have a smaller inside diameter than copper at the same nominal size. Size by inside diameter — don't drop a nominal size when you switch to plastic.
  • Hold hot-water lines to about 5 fps to limit erosion and noise — lower the velocity limit for the hot side.
  • IPC 2021 Appendix E also checks street pressure against elevation and fixture minimums; this tool sizes by velocity and peak demand only. Verify available pressure on a long or borderline run.

How the supply sizing works

Each fixture carries a water-supply-fixture-unit (WSFU) rating from IPC 2021 Table E103.3(2). The tool totals qty × WSFU, then runs that total through three steps:

  • WSFU → GPM on Hunter's demand curve (Table E103.3(3)), interpolated between the tabulated points. Flushometer fixtures use the higher curve.
  • GPM → velocity for each pipe size: V = 0.4085 × GPM / ID² in fps, using the material's real inside diameter.
  • Pick the smallest size that stays at or under the velocity limit (8 fps default for cold copper).

An indicative friction loss (Hazen-Williams, C = 150) is shown alongside for reference. This sizes by velocity and peak demand only; the full Appendix E method also weighs street pressure against elevation and fixture minimums, so verify borderline runs.

Frequently asked

What size water line do I need for a house?
Total the water supply fixture units (WSFU) for every fixture, convert to peak demand in GPM on Hunter's curve, then pick the smallest pipe that keeps velocity at or below your limit. A typical 2.5-bath house totals about 19 WSFU, roughly 13-14 GPM, which needs 1-inch copper — 3/4-inch tops out near 12 GPM at an 8-fps limit.
How do WSFU convert to GPM?
Through Hunter's curve (IPC 2021 Table E103.3(3)), which accounts for the fact that not every fixture runs at once. The curve is steep at low fixture-unit counts and flattens out as it climbs: 10 WSFU is about 8 GPM on the flush-tank curve, 20 WSFU about 14 GPM, and 40 WSFU about 25 GPM. Flushometer valves follow a higher curve.
How does this pick a pipe size?
By velocity. For each candidate inside diameter it computes V = 0.4085 × GPM ÷ ID² in feet per second and takes the smallest pipe that stays at or under your velocity limit — 8 fps by default for cold copper. High velocity causes noise and erosion, which is why the limit, not just the flow, sets the size.
Can I use the same size in PEX or CPVC as copper?
Not always. PEX and CPVC have a smaller inside diameter than copper at the same nominal size — 1-inch PEX is about 0.863 inch ID versus 1.025 inch for Type L copper. Velocity goes up as ID squared, so a run that is fine in 1-inch copper can exceed the limit in 1-inch PEX. Size by inside diameter and don't assume you can drop a size when switching to plastic.
Does this replace a full pressure-drop calculation?
No. IPC Appendix E also checks available street pressure against elevation, meter and pipe pressure loss, and each fixture's minimum flow pressure. This tool sizes by velocity and peak demand, which is the right first pass, but on a long run, a high-rise, or a low-pressure supply you should run the full pressure-loss method or have a plumber verify.

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Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed plumber. See our methodology, sources, and code editions.