Número de Reynolds
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
Re = ρvd/μ
About this calculator
The Reynolds Number calculator determines whether a fluid flow is laminar, transitional, or turbulent. The Reynolds Number (Re) is a dimensionless parameter that relates inertial and viscous forces, calculated by the formula Re = ρvd/μ, where ρ is fluid density, v is flow velocity, d is pipe diameter, and μ is dynamic viscosity. Low Re values (below 2000) indicate laminar flow, while high values (above 4000) indicate turbulent flow; between 2000 and 4000, flow is transitional.
This tool is useful for engineers, technicians, and students working with hydraulic systems, aerodynamics, chemical processes, or any application involving fluid flow in pipes. For example, when designing a pumping system, knowing the flow regime helps predict pressure losses and select appropriate pumps. It is also used in heat exchanger design, water networks, and ventilation systems.
Important precautions: ensure consistent units (SI or compatible system). Density and viscosity vary with temperature and pressure, so use values at operating conditions. The diameter should be the internal pipe diameter. For non-circular ducts, use the hydraulic diameter. Remember that the Reynolds Number is an indicator, but other factors like wall roughness can influence the transition to turbulence.
Frequently asked questions
What does a low Reynolds Number mean?
A low Reynolds Number (below 2000) indicates laminar flow, where fluid layers slide smoothly over each other with little mixing.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity (μ) measures resistance to shear, while kinematic viscosity (ν = μ/ρ) is the ratio of dynamic viscosity to density. The Reynolds Number uses dynamic viscosity.
Can I use this calculator for gases?
Yes, as long as you provide the gas density and viscosity at the flow temperature and pressure conditions.
What if the pipe is not circular?
For non-circular ducts, calculate the hydraulic diameter (4 times the cross-sectional area divided by the wetted perimeter) and use it as the diameter.
Is the Reynolds Number enough to determine if flow is turbulent?
Not entirely. Although it is the main indicator, wall roughness and external disturbances can trigger turbulence earlier. For critical designs, consider other factors.