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Fluid Mechanics Pipe Sizing Calculator

Size water supply pipes by fixture units and flow rate. Calculate copper and PEX pipe diameter for residential plumbing per IPC fixture unit tables.

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Inputs that matter
Flow Rate (Q), Target Velocity
Output to expect
Pipe Sizing Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Fixture Unit Count

Each fixture type contributes drainage or supply fixture units. Supply sizing uses water supply fixture units (WSFU).

Total DFU = Σ Fixture Unit Values (IPC Table 604.3)

Flow Rate from WSFU

Peak demand GPM estimated from total WSFU — not all fixtures flow simultaneously at full rate.

GPM ≈ lookup from WSFU curve (Hunter's curve approximation)

Pipe Diameter Selection

1/2″ ~ 3–5 GPM; 3/4″ ~ 6–10 GPM; 1″ ~ 10–16 GPM at acceptable velocity for residential.

Select pipe size where velocity ≤ 5–8 fps at peak GPM

Updated: July 2026

Example Scenarios

Typical 3-bed home ~25 WSFU peak ~8 GPM. 3/4″ main minimum; 1″ preferred for long runs or high demand.

Lav + tub/shower + toilet branch: 1/2″ adequate for individual bathroom group if length < 30 ft.

Extend 3/4″ trunk; 1/2″ branches to new lav and hose bib. Verify existing main capacity.

FAQ

Most modern homes: 1″ copper or PEX from meter to manifold. Older 3/4″ mains may restrict flow with multiple fixtures open.

Relative demand values per fixture type. Higher WSFU = more simultaneous flow potential. Summed for system sizing.

Same nominal sizes for equivalent flow. PEX ID slightly smaller — follow manufacturer flow charts for PEX specifically.

Long runs increase friction loss — may require upsizing to maintain pressure at remote fixtures.

Yes — trunk and branch method. Main carries cumulative demand; branches sized for local fixtures only.

About Fluid Mechanics Pipe Sizing Calculator

Determine minimum water supply pipe diameter based on fixture unit count and peak demand flow rate. Enter number of fixtures and developed length to select copper, CPVC, or PEX size per IPC/UPC fixture unit methods.