Pipe Flow Reynolds Number and Regime
The Reynolds number behind every friction-factor decision.
Example
You enter
- Mean flow velocity (ft/s) 6
- Inside diameter (in) 2
- Kinematic viscosity (ft²/s) 0.0000121
You get
- Reynolds number 82,645
Checked against published worked examples from: PDHonline (L. A. Sequeira).
Details, formula, and sources
ScopeRe = V D / nu sorts pipe flow into laminar (Re < 2,300), transitional, and turbulent (Re > 4,000). 60 F water at 6 ft/s in a 2 in line runs Re 82,600 - firmly turbulent, as nearly all trade piping is, which is why the Hazen-Williams and Colebrook forms apply; slow it to a crawl and Re 275 laminar, where f = 64/Re and loss is linear in velocity. Enter the kinematic viscosity for the fluid and temperature. An engineering aid; the fluid property data govern.
Formula or tableRe = V (D/12) / nu; regime = Re < 2,300 laminar, <= 4,000 transitional, else turbulent.
Source and editionThe Reynolds number Re = V D / nu (= rho V D / mu) and the standard pipe-flow transition bands, with a 60 degF water kinematic viscosity of about 1.21e-5 ft^2/s, a standard fluid-mechanics result, by name.
Free accessThe Reynolds-number definition and the pipe-flow transition bands are public results in any fluid-mechanics reference.
What governsEstimate. AHJ and licensed professional govern.
API field namesv_fps, d_in, nu, re
- Reynolds number Re = V D / nu, velocity times diameter over kinematic viscosityfluid mechanics
- Regime bands laminar below ~2,300, transitional ~2,300 to 4,000, turbulent above ~4,000pipe-flow convention
- Viscosity entered per fluid and temperature; ~1.21e-5 ft^2/s for 60 degF waterfluid property tables