What is the Reynolds number?

The Reynolds number (Re) is a dimensionless quantity that compares inertial forces to viscous forces in a flowing fluid. It predicts whether flow will be smooth and ordered (laminar) or chaotic (turbulent).

What are the flow regime thresholds?

For internal pipe flow: Re 4000 is turbulent. Thresholds differ for external flows such as flow over flat plates.

What units should I use?

This calculator uses SI units: density in kg/m³, velocity in m/s, length in m, and dynamic viscosity in Pa·s (which equals kg/(m·s)). All inputs must be consistent in units for a valid result.

What is the characteristic length?

For pipe or duct flow, the characteristic length is the hydraulic diameter. For flow over a flat plate it is the plate length. For a sphere it is the sphere diameter. Choose the dimension most relevant to your geometry.

How is dynamic viscosity different from kinematic viscosity?

Dynamic viscosity (μ) is the absolute measure of a fluid's resistance to flow, in Pa·s. Kinematic viscosity (ν) equals μ/ρ, in m²/s. This calculator uses dynamic viscosity; divide μ by ρ if you only know kinematic viscosity.

Reynolds Number Calculator

Calculate the Reynolds number (Re = ρVD/μ) and classify fluid flow as laminar, transitional, or turbulent.

Fluid Parameters

Quick Presets

Fluid Density (ρ) kg/m³

Flow Velocity (V) m/s

Characteristic Length / Diameter (D) m

Dynamic Viscosity (μ) Pa·s

Enter fluid parameters and click Calculate

Re = ρ × V × D / μ

Summary

The Reynolds Number Calculator computes Re = ρVD/μ, where ρ is fluid density, V is flow velocity, D is characteristic length (diameter), and μ is dynamic viscosity. The result is used to predict whether a fluid flow is laminar (Re < 2300), transitional (2300–4000), or turbulent (Re > 4000).

How it works

Enter fluid density (ρ) in kg/m³.
Enter flow velocity (V) in m/s.
Enter characteristic length or pipe diameter (D) in meters.
Enter dynamic viscosity (μ) in Pa·s (kg/m·s).
Click "Calculate" to get the Reynolds number and flow regime classification.
Use the preset fluid buttons to auto-fill common fluid properties.

Use cases

Determine whether pipe flow is laminar or turbulent in HVAC and plumbing design.
Analyze aerodynamic behavior around aircraft wings and car bodies.
Model chemical reactor mixing efficiency in process engineering.
Validate computational fluid dynamics (CFD) simulations.
Estimate heat and mass transfer rates in heat exchanger design.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu