Pipe diameter is selected from a system calculation. Velocity alone cannot prove that pressure will be adequate.
1. Define the design flow
List the outlets that may operate together and assign a realistic flow to each. Peak flow is different from daily water use: a home may consume a modest total volume while still needing several fixtures for a short period.
For a private supply, compare the instantaneous demand with the pump, pressure tank and any intermediate storage. A low-yield source can still serve a home when storage and controls are designed around the demand pattern.
Penn State Extension · Estimating private water needs ↗2. Work with internal diameter
Nominal pipe size is not necessarily its bore. Wall thickness varies by material and pressure class, so obtain the manufacturer’s internal diameter before calculating velocity or friction loss.
Velocity is flow divided by internal area. A smaller bore raises velocity quickly because area changes with diameter squared. Use the calculator as a screening step, then calculate friction through the entire route.
3. Add pressure losses and elevation
Straight pipe, fittings, valves, filters and treatment equipment all consume pressure. Elevation also matters: water delivered uphill requires additional head before any useful fixture pressure remains.
Check the operating point against the pump curve, not only its maximum head or maximum flow. Those two catalogue values normally occur at different conditions. Allow for dirty filters and plausible simultaneous use.
4. Verify the installed system
Record static pressure, dynamic pressure and flow at useful test points. Compare them while representative fixtures run. Unexpected loss may come from a partly closed valve, undersized fitting, fouled filter or a pipe smaller than shown on the plan.
Local plumbing rules can impose diameter, velocity, material and backflow requirements. Have the final design checked where failure could damage property or compromise drinking water.
Inputs for a pipe-sizing worksheet
| Input | Why it matters | Evidence |
|---|---|---|
| Simultaneous flow | Sets the design duty | Fixture schedule or measured flow |
| Internal diameter | Determines area and velocity | Manufacturer data |
| Length and fittings | Determine friction loss | Measured route |
| Elevation change | Adds or removes static head | Level survey |
| Required outlet pressure | Defines usable result | Equipment and local requirements |
Worked example
A quick velocity check
A pipe has a 25 mm internal diameter and carries 20 L/min.
Flow = 0.000333 m³/s Area = π × 0.025² / 4 = 0.000491 m² Velocity ≈ 0.68 m/s
The calculation describes velocity only. Pressure loss, pump curve, surge and local rules still need separate checks.
What to have ready
- Simultaneous fixture-flow estimate
- Actual pipe internal diameter
- Route length, fittings and elevation
- Pump curve and required outlet pressure
Common mistakes
- Using daily litres as the peak flow.
- Using nominal size as internal diameter.
- Ignoring filters, fittings and elevation.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.