Free Calculator
Pipe Size Calculator
Find the right water-supply pipe diameter for a target flow rate — sizing to keep velocity in a safe, quiet range across copper, PEX, CPVC, and galvanized steel.
See SubcontractorHub for PlumbingSize Your Water Supply Pipe
Enter your design flow rate, pipe material, and velocity limit to see the smallest nominal size that stays in a safe range.
Use peak simultaneous demand, not the sum of every fixture.
Copper is best kept near 5 ft/s to limit erosion-corrosion; PEX and CPVC tolerate up to 8 ft/s.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on typical averages and should be used for general planning purposes only. They are not a substitute for a professional assessment or written contractor quote. Actual costs, savings, and results will vary significantly based on your specific circumstances, local market conditions, equipment choices, and contractor pricing. Always confirm any estimate with a licensed contractor, financial advisor, or qualified professional before making purchasing or financial decisions.
How Water Supply Pipe Size Is Determined
Velocity is the number that drives pipe size
For a known flow rate, the pipe's internal diameter sets the water velocity. Most plumbing guidance keeps design velocity at or below 5 ft/s for long, quiet service life, with 8 ft/s as a practical maximum. The calculator above steps through standard nominal sizes and picks the smallest one whose velocity stays at or under your chosen limit:
| Nominal size | Approx. flow at 5 ft/s | Approx. flow at 8 ft/s |
|---|---|---|
| 1/2" | ~3 GPM | ~5 GPM |
| 3/4" | ~6 GPM | ~10 GPM |
| 1" | ~11 GPM | ~17 GPM |
| 1-1/4" | ~16 GPM | ~26 GPM |
| 2" | ~42 GPM | ~67 GPM |
Figures use approximate internal diameters and round for readability; the calculator computes exact velocity from A = π/4 × (ID/12)² and velocity = (GPM × 0.002228) / A.
Why velocity limits matter: noise, erosion, water hammer
Pushing water too fast through a pipe creates audible flow noise, accelerates erosion and erosion-corrosion of the pipe wall (a well-known concern in copper), and raises the energy released as water hammer when a valve or fixture shuts quickly. Keeping velocity down protects the system's long-term life and keeps it quiet — which is why 5 ft/s is a common design target even when a material could technically handle more. If you manage plumbing jobs end to end, Plumbing CRM keeps the material and sizing decisions documented alongside the proposal and job record.
Material changes the recommended velocity
Copper is more sensitive to erosion-corrosion, especially with hot water, so a conservative ~5 ft/s design velocity is common even though its cited maximum is around 8 ft/s. PEX and CPVC are smoother and more tolerant, comfortably running near 8 ft/s, while galvanized steel is typically kept closer to 6 ft/s. Internal diameters also vary by material and schedule, which shifts the velocity you get at the same nominal size — always check the actual ID for the product you are installing.
This is a single-run check, not WSFU code sizing
Whole-house and branch sizing under the plumbing code uses Water Supply Fixture Units (WSFU), which map simultaneous fixture demand to pipe size through code tables that also account for developed length and pressure loss. This calculator is a fast velocity check for a single run at a known design flow rate — helpful for a specific branch or service line, but not a replacement for a full WSFU calculation. For whole-house work, size with the fixture-unit method.
Always confirm against local plumbing code
- Local code (IPC, UPC, or a state amendment) governs minimum pipe sizes and methods
- Developed length, fittings, and elevation change all add pressure loss this tool ignores
- Available supply pressure at the meter or well sets a real-world ceiling on flow
- Hot-water runs in copper are often held to a lower velocity than cold
- A licensed plumber should sign off on any sizing used for a real installation
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Book a Free DemoFrequently Asked Questions
What size pipe do I need for a given GPM?
Pipe size for a target flow rate depends on keeping water velocity in a safe range — usually 5 ft/s for quiet, long-lasting systems and up to 8 ft/s maximum. As a rough guide, 1/2" handles up to about 3–5 GPM, 3/4" up to about 8–12 GPM, and 1" up to about 15–20 GPM at a 5–8 ft/s target. This tool picks the smallest nominal size that keeps velocity at or below your chosen limit.
Why does water velocity in a pipe matter?
High velocity causes noise, accelerates erosion and erosion-corrosion of pipe walls (especially in copper), and increases water hammer risk when valves close. Most plumbing guidance caps design velocity around 5 ft/s for cold water and lower for hot water in copper to protect long-term pipe life. Oversizing wastes material and money; undersizing creates noise and premature failure.
Does pipe material change the recommended size?
Yes. Copper is more sensitive to erosion-corrosion, so a lower design velocity (~5 ft/s) is common even though its max is often cited at 8 ft/s. PEX and CPVC tolerate higher velocities (~8 ft/s) and have smoother, quieter flow, while galvanized steel is typically kept nearer 6 ft/s. Internal diameters also differ slightly by material and schedule, which shifts velocity at the same nominal size.
Is this the same as code fixture-unit sizing?
No. Whole-house and branch sizing under the plumbing code uses Water Supply Fixture Units (WSFU) mapped to pipe size through code tables that account for simultaneous demand, developed length, and pressure loss. This calculator is a single-run velocity check for a known design flow rate — useful for a specific branch or service line — not a substitute for a full WSFU calculation or local code compliance.
What flow rate should I design for?
Use the peak simultaneous demand the run must serve, not the sum of every fixture. A single shower is roughly 2–2.5 GPM, a kitchen sink 1.5–2.2 GPM, and a hose bib 5+ GPM. For a branch feeding several fixtures, estimate how many run at once during peak use. When in doubt, a licensed plumber can convert fixture counts to a design GPM using code methods.
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Get a Free DemoAll calculations are ballpark estimates based on approximate pipe internal diameters and single-run velocity limits, and should be verified against local plumbing code and with a licensed plumber before making any installation or financial decisions.