Plumbing Pipe Size Calculator
Estimate the minimum internal pipe diameter needed to carry a selected water flow without exceeding an entered design velocity. This is a preliminary hydraulic sizing check, not a plumbing-code or pressure-loss design.
Default project preview
1.21 in
Required flow area
1.16 in²
Proposed internal diameter
1 in
Velocity in proposed bore
7.35 ft/s
Velocity margin
-47.1%
Calculator
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Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Design flow rate
Enter this project assumption in gpm. The current imperial default is 18 gpm.
Maximum design velocity
Enter this project assumption in ft/s. The current imperial default is 5 ft/s.
Proposed internal diameter
Enter this project assumption in in. The current imperial default is 1 in.
Reference run length
Enter this project assumption in ft. The current imperial default is 80 ft. Minimum: 0.
How to read the outputs
Required flow area
flow ÷ target velocity. Default preview: 1.16 in².
Proposed internal diameter
user-entered bore for comparison. Default preview: 1 in.
Velocity in proposed bore
above entered velocity limit. Default preview: 7.35 ft/s.
Velocity margin
negative means the proposed bore is too small for this velocity target. Default preview: -47.1%.
Reference run length
shown for downstream pressure-loss checks. Default preview: 80 ft.
Hydraulic sizing status
pressure loss and code checks still required. Default preview: Increase internal bore.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
For incompressible flow, Q = A × v. Required cross-sectional area A = Q ÷ v, and required internal diameter d = √(4A ÷ π). Unit conversions are applied before the calculation.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: August 19, 2026
Perform a preliminary hydraulic bore check from design water flow and an entered maximum velocity. The calculator derives minimum internal diameter from Q = A × v and compares that with a proposed actual internal diameter; it does not select a code-approved nominal pipe size.
Calculation steps
- 1Establish a defensible design flow for the section of piping.
- 2Choose a project design velocity criterion rather than assuming the default is a code rule.
- 3Calculate flow area as flow divided by velocity, then derive the equivalent circular internal diameter.
- 4Compare with the actual bore of a candidate pipe and continue with pressure-loss and code sizing checks.
Worked numerical example
For 18 gpm with a maximum velocity of 5 ft/s, the velocity relationship requires roughly a 0.97 in internal bore. A proposed 1.00 in actual internal diameter carries the same flow at about 4.6 ft/s, so it passes this velocity-only check.
Purchasing and rounding
Do not purchase from this diameter alone. Nominal size, schedule, SDR and tubing standard all change actual bore, and fittings/valves affect pressure loss. Final material selection should use manufacturer dimensions and the applicable plumbing design method.
Interpret the result
A passing velocity result means only that the proposed internal bore carries the entered flow at or below the selected velocity. It says nothing by itself about residual pressure, fixture-unit compliance, hot-water recirculation or water-hammer control.
Page-specific assumptions
- Steady incompressible water flow.
- Circular unobstructed internal bore.
- Entered flow represents the design flow for the pipe section.
- Velocity criterion is selected externally for the actual system.
Common mistakes
- Treating nominal diameter as actual internal diameter.
- Using fixture count directly as gpm without a demand method.
- Ignoring developed length, elevation and fitting losses.
- Presenting a velocity-only result as plumbing-code sizing.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
Related tools
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Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Plumbing Pipe Size Calculator questions
Is this enough to choose a plumbing pipe size?+
No. It only checks diameter against flow velocity. Final sizing must also verify pressure loss, fixture demand, developed length, elevation and the applicable plumbing code.
Why ask for internal diameter instead of nominal pipe size?+
Flow area is controlled by the actual bore. Two pipes with the same nominal size can have different internal diameters.
What velocity should I use?+
Use the design criterion appropriate to the system, material, noise and erosion concerns. Do not treat the default value as a code requirement.