Pipe Flow & Velocity Calculator
Velocity from flow and pipe bore (or flow from velocity), Reynolds number with the regime named, velocity head, and the diameter that meets a target velocity.
The velocity in the pipe, whether the flow is laminar or turbulent for your fluid, and the bore you would need to hit a velocity you choose — all from continuity, nothing assumed.
Example: 10 m³/h through a 50 mm bore: 1.41 m/s, velocity head 0.10 m; with water at 998 kg/m³ and 1.002 mPa·s, Re ≈ 70,000 (turbulent). For 1 m/s the bore would need to be 59.5 mm.
Continuity is exact.
The regime is a label.
What the velocity, the Reynolds number and the suggested diameter mean, and what this page deliberately does not compute.
Velocity and flow
For a full pipe the volume flow is the cross-section times the average velocity: Q = A × v with A = πD²/4 on the inside diameter. Give either quantity and the other follows; the flow is also listed in the common units. Use the bore, not the nominal size — a DN50 or 2-inch pipe has a bore that depends on its wall thickness (the Pipe Weight & Schedule page lists them).
Reynolds number and regime
Re = ρvD/μ compares inertia to viscosity using the density and dynamic viscosity you enter (the water figures shown at first are a labelled sample at about 20 °C; other fluids and temperatures differ). The regime label follows the usual pipe-flow criterion — laminar below 2,300, turbulent above 4,000, transitional between — which is a textbook convention, not a sharp physical boundary; in the transitional band the flow can be either, and a note says so.
Velocity head and target diameter
The velocity head v²/2g is the height of fluid equivalent to the kinetic energy of the flow; it appears in pump-head sums and in minor-loss formulas (loss = K × v²/2g, with K from your fitting data). If you enter a target velocity, the diameter shown is the bore that would give exactly that velocity at the same flow — D = √(4Q ÷ πv). Pick the target yourself: acceptable velocities depend on the fluid, the material and noise or erosion limits, and this page sets none.
Not computed here
Friction loss (Darcy–Weisbach) needs a friction factor from the Reynolds number and the pipe's relative roughness; roughness values are material data this page does not carry. Pressure drop through fittings, partially full pipes, gases at changing pressure and non-Newtonian fluids are likewise out of scope. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Çengel, Y. A. & Cimbala, J. M., Fluid Mechanics: Fundamentals and Applications — §8-2 laminar and turbulent flows (Re 2,300 / 4,000 criterion), continuity
- NIST Special Publication 811 (2008), Guide for the Use of the SI — unit conversion factors
Last reviewed 19 September 2026. How results are checked: How we verify.