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.
Built for Fluid Mechanics1 documented formulaFocused single-task workflowPrivate in your browser
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A quick decision brief for this specific tool
Inputs that matter
Flow Rate (Q), Target Velocity
Output to expect
Pipe Sizing Calculator
How it works
1 documented formula across 1 calculation mode
- Keep 2 bounded inputs within the limits shown beside the fields.
- Confirm that every value uses the unit and time basis shown before comparing scenarios.
- Change one input at a time to see which assumption materially changes the result.
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Reference & details
How it works
Updated July 2026
How it works
Updated July 2026Fixture 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 GPMUpdated: 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.