Calculators
Reynolds Number Calculator
Calculate dimensionless Reynolds number from density, speed, characteristic length, and dynamic viscosity.
Compare inertial and viscous effects with Re = ρvL/μ.
Reynolds number Re = ρvL/μ compares inertial and viscous effects. Enter fluid density ρ in kg/m³, speed magnitude v in m/s, characteristic length L in meters, and dynamic viscosity μ in Pa·s. For 1000 kg/m³, 1 m/s, 0.05 m, and 0.001 Pa·s, Re = 50000. For circular pipe flow use internal diameter and mean speed. Approximate smooth-pipe ranges are Re < 2300 laminar, 2300 through 4000 transitional, and above 4000 turbulent. These are contextual estimates: geometry, roughness, disturbances, and entrance effects influence transition. For external flow or other geometries choose the appropriate characteristic length and do not use the pipe classification. This model assumes a Newtonian fluid with constant properties. Use dynamic viscosity, not kinematic viscosity; 1 cP = 0.001 Pa·s. Density, length, and viscosity must be positive; speed can be zero. Nonzero inputs range from 1e-12 to 1e12. Results use browser floating-point arithmetic rounded to 12 significant digits. Inputs remain on your device.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What is the Reynolds number formula?
Re = ρvL/μ, using density in kg/m³, speed in m/s, length in m, and dynamic viscosity in Pa·s. The result is dimensionless.
What length should I enter?
Use internal diameter for a circular pipe. Other geometries need their own characteristic length, such as hydraulic diameter for a noncircular duct.
What is the worked example?
Density 1000 kg/m³, speed 1 m/s, diameter 0.05 m, and viscosity 0.001 Pa·s give Re = 50000.
Do the flow classifications apply everywhere?
No. The approximate thresholds 2300 and 4000 refer to smooth circular pipe flow. Geometry, roughness, and disturbances affect transition; external flow has different criteria.
Is dynamic viscosity the same as kinematic viscosity?
No. Dynamic viscosity μ is measured in Pa·s; kinematic viscosity ν is measured in m²/s and equals μ/ρ. Convert dynamic viscosity from cP by multiplying by 0.001.
Are inputs uploaded?
No. Calculation and copying run locally in your browser. Reset restores the example.