What is the Reynolds number?

The Reynolds number is a dimensionless ratio of inertial to viscous forces: Re = vD/ν. It predicts whether flow will be laminar ( 4000).

What determines laminar vs turbulent flow?

Re 4000: turbulent (chaotic). These thresholds apply to pipe flow.

What is kinematic viscosity?

Kinematic viscosity ν = μ/ρ, with units m²/s or cSt (1 cSt = 10⁻⁶ m²/s). Water at 20°C: ~1 cSt. Hydraulic oil ISO VG 46 at 40°C: ~46 cSt.

Why does Reynolds number matter?

Re determines friction factor, heat transfer coefficient, and mixing behavior. It's critical for pipe sizing, pump selection, and pressure drop calculations.

What is hydrodynamic entry length?

Entry length is the distance from pipe inlet before the flow profile becomes fully developed. Laminar: Le ≈ 0.06·Re·D. Turbulent: Le ≈ 4.4·Re^(1/6)·D.

Free Engineering Tool

Reynolds Number Calculator

Calculate Reynolds number from velocity or flow rate. Visual laminar/transition/turbulent indicator. Entry length calculation and common fluid presets.

Re = vD/ν Laminar / Turbulent Entry Length Fluid Presets

Results

Reynolds Number

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Flow Regime

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Flow Velocity

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Critical Velocity (Re=2300)

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Entry Length Le

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Entry Length / Diameter

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Laminar2300Transition4000Turbulent

Flow Regime Visualization

Reynolds Number

The Reynolds number is the ratio of inertial forces to viscous forces in a flowing fluid:

v — flow velocity (m/s)
D — pipe inner diameter (m)
ν — kinematic viscosity (m²/s); 1 cSt = 10⁻⁶ m²/s
ρ — fluid density (kg/m³)
μ — dynamic viscosity (Pa·s)

Flow Regime Thresholds

Reynolds Range	Regime	Characteristics
Re < 2,300	Laminar	Smooth, orderly layers; friction factor f = 64/Re; predictable
2,300 – 4,000	Transition	Unstable; intermittent turbulence; avoid designing here
Re > 4,000	Turbulent	Chaotic; higher friction; use Colebrook-White for f

Hydrodynamic Entry Length

The distance from the pipe inlet until the velocity profile becomes fully developed:

Common Fluid Viscosities

Fluid	Temperature	ν (cSt)	ρ (kg/m³)
Water	20°C	1.004	998
Water	40°C	0.658	992
Water	80°C	0.365	972
Hydraulic Oil ISO VG 32	40°C	32	870
Hydraulic Oil ISO VG 46	40°C	46	870
Hydraulic Oil ISO VG 68	40°C	68	880
Air	20°C, 1 atm	15.6	1.204
Engine Oil SAE 30	40°C	100	880

Practical Example

Example — Hydraulic System

Given: v = 2 m/s, D = 25 mm, Oil ISO VG 32 at 40°C (ν = 32 cSt)

Re = 2 × 0.025 / (32 × 10⁻⁶) = 1,563

Regime: Laminar (Re < 2300)

Entry length: Le = 0.06 × 1563 × 0.025 = 2.34 m

Critical velocity for Re = 2300: v_crit = 2300 × 32 × 10⁻⁶ / 0.025 = 2.94 m/s

💡 Tip: In hydraulic systems with oil (ν = 30–100 cSt), flow is almost always laminar due to high viscosity. Water systems typically have turbulent flow because ν ≈ 1 cSt.

Reynolds Number Calculator | Free Online Re Tool | Vibromera

Published by Nikolai Shelkovenko on February 15, 2026 March 5, 2026

Results

Reynolds Number

Flow Regime Thresholds

Hydrodynamic Entry Length

Common Fluid Viscosities

Practical Example