Choosing a pipe inside diameter from flow
A velocity-based diameter is a screening result. It narrows candidate sizes but does not select a pipe schedule, pressure rating or economic optimum. Every candidate must be checked for losses and the applicable project criteria.
Worked numbers are illustrative. Use your own operating conditions and documented material/process data.
Required inputs
- Required volumetric flow and a justified target mean velocity.
- Actual product inside diameters for candidate pipe sizes.
- Fluid properties, length and fittings for the downstream pressure-loss check.
Governing equations
Arequired = Q/vtarget
Drequired = √[4Q/(πvtarget)]
vactual = 4Q/(πDactual²)
Variables and units
- Q: m³/s; v: m/s; A: m²; D: m.
- vtarget is a project input, not a universal pass/fail limit.
Assumptions
- Circular full-bore liquid flow and one velocity target.
- The worked example’s 2 m/s target is illustrative, not a design recommendation.
Calculation workflow
- Convert the required flow to m³/s.
- Calculate a theoretical inside diameter from the entered target.
- Choose an actual product ID and recalculate velocity, Reynolds number and losses rather than rounding DN alone.
Worked example
- Q = 25 m³/h = 0.006944444 m³/s; vtarget = 2 m/s.
- Drequired = 0.066490 m = 66.490 mm.
- A candidate with 77.9 mm actual ID gives vactual = 1.457044 m/s.
Limitations
- Pipe rating, wall thickness, corrosion allowance, support spacing and minimum transport velocity are not determined here.
- Hydraulic ID must come from the actual product record; do not manufacture a schedule table from nominal DN labels.
Common mistakes
- Entering 25 as m³/s when the duty is 25 m³/h.
- Using OD as the flow diameter.
- Calling a target-velocity result a code-approved minimum size.
Practical interpretation
Increasing actual diameter lowers velocity at fixed flow. However, a larger pipe is not automatically the correct selection: capital cost, fluid transport needs, solids, corrosion, available pressure and pumping energy can matter. The sizing result should travel with its target and assumptions into the report.
Related formulas
The governing equations, units and calculation steps above provide the formula basis for this guide. The related workspace also shows its calculation trace.
Pipe area and velocity references →Related workspace
Open Fluid Systems →Sources and reference basis
Equations and worked examples are independently explained here. References reviewed 2026-10-17; no copyrighted standard tables are reproduced.