ℹ About This Calculator
The Reynolds number is the single most important dimensionless quantity in fluid mechanics — it predicts whether flow in a pipe or duct is smooth and orderly (laminar) or chaotic and mixing (turbulent), which in turn governs friction loss, heat transfer and pressure drop. This calculator computes the Reynolds number from the fluid velocity, the characteristic length (pipe diameter) and the fluid's kinematic viscosity, and classifies the flow regime using the standard transition thresholds. It works in US units (ft/s, inches) and is the starting point for pipe friction (Moody chart), heat-exchanger and process-flow design.
The Reynolds number is the ratio of inertial forces to viscous forces in a flowing fluid. When viscous forces dominate (low Re) the flow is laminar; when inertial forces dominate (high Re) it becomes turbulent. For internal pipe flow the widely used thresholds are Re < 2,300 laminar and Re > 4,000 fully turbulent, with a transitional band between. The regime determines which friction-factor relationship applies on the Moody chart and profoundly affects pressure drop, mixing and heat transfer.
Frequently Asked Questions
What is the Reynolds number?
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The Reynolds number (Re) is a dimensionless quantity equal to the ratio of inertial forces to viscous forces in a moving fluid, Re = V·D/ν. It predicts the flow regime: a low Reynolds number means viscous forces dominate and the flow is smooth and laminar, while a high Reynolds number means inertial forces dominate and the flow is turbulent. It is the key parameter for determining friction loss, heat transfer and mixing in pipes and ducts.
What Reynolds number is laminar vs turbulent?
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For flow in a pipe, Re below about 2,300 is laminar (smooth, orderly flow), Re above about 4,000 is turbulent (chaotic, mixing flow), and the range from 2,300 to 4,000 is transitional, where the flow can switch unpredictably. These thresholds apply to internal pipe flow; flow over flat plates and other geometries have different critical values.
How do I calculate the Reynolds number for water in a pipe?
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Use Re = V·D/ν, where V is the velocity in ft/s, D is the pipe inside diameter in feet, and ν is the kinematic viscosity (about 1.21×10⁻⁵ ft²/s for water at 60°F). For example, water at 5 ft/s in a 2-inch (0.167 ft) pipe gives Re = 5 × 0.167 / 1.21×10⁻⁵ ≈ 68,900, which is turbulent. Remember to convert the pipe diameter from inches to feet.
Why does the flow regime matter?
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The flow regime determines the friction factor and therefore the pressure drop, as well as the heat-transfer coefficient and the degree of mixing. Laminar flow has a friction factor of 64/Re and low, predictable losses; turbulent flow follows the Colebrook/Moody relationship with higher losses that depend on pipe roughness. Nearly all practical water and air piping runs turbulent, which is why the Moody chart and turbulent friction equations dominate pipe design.
What is kinematic viscosity?
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Kinematic viscosity (ν) is the ratio of a fluid's dynamic viscosity to its density, with units of ft²/s (or m²/s). It represents how readily a fluid resists shear relative to its inertia. Water at 60°F has ν ≈ 1.21×10⁻⁵ ft²/s, dropping as temperature rises; air at 60°F is about 1.58×10⁻⁴ ft²/s. Because viscosity changes with temperature, the Reynolds number and flow regime shift with fluid temperature.
Does pipe diameter affect the Reynolds number?
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Yes — the Reynolds number is directly proportional to the characteristic length, which for pipe flow is the inside diameter. A larger pipe at the same velocity has a higher Reynolds number and is more likely to be turbulent. This is why the diameter must be included, and why very small tubes and capillaries can remain laminar at velocities that would be turbulent in a large pipe.
Is this Reynolds number calculator accurate?
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It applies the standard Re = V·D/ν definition with the accepted laminar and turbulent thresholds for pipe flow, so it is reliable for determining the flow regime and feeding pipe-friction and heat-transfer calculations. For precise work, use the exact kinematic viscosity at your fluid's temperature, and remember that the transitional band (2,300-4,000) is inherently uncertain.
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