Reynolds Number Calculator

• Velocity or flow rate. Enter the mean velocity directly, or a volumetric flow rate and let the page convert it with V = Q/A, A = π·D²/4. Both routes give the same Re.
• Pipe diameter D. The internal (bore) diameter, not the nominal size or the outside diameter — a 100 mm nominal Schedule 40 pipe has a 102.3 mm bore. For a non-circular duct use the hydraulic diameter D_h = 4·A/P (four times the flow area over the wetted perimeter).
• Density ρ. At the operating temperature; for a gas, at the operating pressure too (density scales with absolute pressure).
• Dynamic viscosity μ. 1 cP = 1 mPa·s = 0.001 Pa·s. If you have the kinematic viscosity ν (m²/s or cSt), Re = V·D/ν — the density cancels, so either pair works.
• Fluid presets. Fixed ρ and μ at the stated temperature, for a quick check. For other temperatures pick Custom and enter properties at the operating condition — viscosity is the input that moves Re the most.
• Flow regime. Laminar below Re 2,000, turbulent above 4,000, transitional between. The bands are the conventional pipe-flow thresholds; the actual transition point depends on inlet disturbances and pipe roughness.

• Full circular pipe with the internal diameter as the characteristic length; for ducts and annuli use the hydraulic diameter, and expect the transition thresholds to be approximate there.
• Newtonian fluid with uniform properties at the operating temperature (and pressure for gases). Slurries, polymer solutions and other non-Newtonian fluids need an apparent viscosity and a modified Reynolds number.
• The regime thresholds (2,000 / 4,000) are the pipe-flow convention; some texts quote a critical Re of 2,300. Laminar flow can persist to much higher Re in a carefully undisturbed pipe, and rough or disturbed inlets trip turbulence earlier.
• Re alone does not give the pressure drop. It selects the friction-factor relation — f = 64/Re laminar, Colebrook-White turbulent — which also needs the pipe roughness; use the friction loss calculator for the drop itself.
• Single-phase flow. Two-phase and free-surface (open-channel) flows use different Reynolds definitions and thresholds.
• For very small pipes or very viscous fluids the laminar regime is normal and not a problem in itself — but fittings' K-factors and valve Cv data are turbulent-basis and over- or under-state losses there.

The calculator and the SimuPipe solver share the same Reynolds-number and friction-factor code — the solver evaluates Re on every pipe of a network to pick the regime and friction factor. Its accuracy is documented case by case on the validation page (53 published cases, each with its per-pipe Reynolds number in the calculation trace).

None of these methods are ours — check them at the source.

• Reynolds, O. (1883). "An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels." Philosophical Transactions of the Royal Society, 174, 935–982. doi:10.1098/rstl.1883.0029 — the original transition experiments and the dimensionless group.
• Moody, L. F. (1944). "Friction factors for pipe flow." Transactions of the ASME, 66, 671–684 — the regime bands and friction-factor chart the thresholds come from.
• Colebrook, C. F. (1939). "Turbulent flow in pipes…" Journal of the Institution of Civil Engineers, 11(4), 133–156. doi:10.1680/ijoti.1939.13150
• Crane Co. Flow of Fluids Through Valves, Fittings, and Pipe, Technical Paper No. 410 — practical thresholds, fluid properties and the turbulent basis of fitting data.
• White, F. M. Fluid Mechanics, McGraw-Hill — derivation, hydraulic diameter, and the discussion of transition.