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Mechanical Engineering2D

Reynolds Number Calculator

Calculate Reynolds number to determine flow regime (laminar, transitional, turbulent) in pipes.

Inputs
Enter your values below

Water at 20°C ≈ 0.001 Pa·s

Result
Reynolds Number (Re)
—
Enter values and click Calculate
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Formula
density × velocity × diameter ÷ viscosity
Result unit: dimensionless

Re = ρ × v × D / μ. Density (kg/m³), velocity (m/s), diameter (m), dynamic viscosity (Pa·s). Re < 2300 laminar, 2300-4000 transitional, >4000 turbulent.

Introduction

Reynolds number is the dimensionless ratio of inertial to viscous forces in a fluid. It determines whether flow is laminar (Re < 2300), transitional (2300-4000), or turbulent (Re > 4000). Essential for friction factor selection and pressure drop calculations.

How This Calculator Works

Re = ρ × v × D / μ. Density (kg/m³), velocity (m/s), diameter (m), dynamic viscosity (Pa·s). Re < 2300 laminar, 2300-4000 transitional, >4000 turbulent.

Step-by-step process:

  1. Enter your input values in the calculator above
  2. The engine converts all inputs to SI base units (meters, kg, Pa)
  3. The formula is evaluated: density * velocity * diameter / viscosity
  4. Result is formatted with the appropriate unit and precision

Calculation Example

Water at 2 m/s in 100mm pipe: Re = 200,000 → turbulent flow

Inputs: density=1000, velocity=2, diameter=100, viscosity=0.001
Result: 200000

Unit Conversion for Reynolds Number

**Viscosity units**

- 1 cSt = 1 mm²/s = 1×10⁻⁶ m²/s

- 1 cP = 0.001 Pa·s = 0.001 kg/(m·s)

- 1 Pa·s = 1000 cP

**Density units**

- 1 kg/m³ = 0.06243 lb/ft³

- 1 lb/ft³ = 16.018 kg/m³

**Length units**

- 1 inch = 25.4 mm

- 1 m = 3.28084 ft

Re = ρVD/μ requires consistent SI units: density in kg/m³, velocity in m/s, diameter in m, dynamic viscosity in Pa·s. Convert all inputs to SI before computing.

Engineering Applications

  • •Flow regime determination
  • •Friction factor calculation
  • •Pressure drop estimation
  • •Heat transfer design

Frequently Asked Questions

What is a good Reynolds number for pipe flow?

For most industrial applications, turbulent flow with Re > 10,000 is desirable for good mixing and heat transfer. Laminar flow (Re < 2300) is common in very viscous fluids or small pipes.

Why does Reynolds number matter?

Re determines which pressure drop equation to use and is required to find the friction factor in the Darcy-Weisbach equation.

How do I convert kinematic viscosity units?

1 cSt = 1 mm²/s = 1×10⁻⁶ m²/s. To convert cSt to m²/s multiply by 1e-6. Dynamic viscosity μ (Pa·s) = kinematic viscosity ν (m²/s) × density ρ (kg/m³); 1 cP = 0.001 Pa·s.

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Engineering Disclaimer: Calculations are for reference and educational purposes only. Always verify results independently for engineering design. See full disclaimer.
Reviewed by: Industrial Engineering Team
References: ASME B31.3, ASTM A36, Perry's Chemical Engineers' Handbook