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Fluid Mechanics

Reynolds Number Calculator

The Reynolds number is a dimensionless quantity that predicts the flow regime in a pipe. It compares inertial forces to viscous forces. This calculator supports both dynamic viscosity and kinematic viscosity formulations for internal circular pipe flow.

TypeInteractive calculator — separate from unit conversions

Calculator

Result
Reynolds number50000
Flow regimeTurbulent

Formulas

Using dynamic viscosity
Re = ρ × v × D / μ
Using kinematic viscosity
Re = v × D / ν

Diagram

Reynolds Number: Flow Regimes in a PipeLaminarTrans.TurbulentRe = 2300Re = 4000Re = ρvD/μInternal circular pipe flow

Worked example

Water flows in a pipe: ρ = 1000 kg/m³, v = 1 m/s, D = 0.05 m (50 mm), μ = 0.001 Pa·s. What is the Reynolds number?

  1. 01Re = ρ × v × D / μ
  2. 02Re = 1000 × 1 × 0.05 / 0.001
  3. 03Re = 50,000
  4. 04Re > 4000 → Turbulent flow
Result

The Reynolds number is 50,000. The flow regime is turbulent.

FAQ

Does this calculate pressure drop?
No. Pressure drop requires friction factor, pipe length, and fittings data. This calculator only computes the Reynolds number.
Does this calculate friction factor or use the Moody chart?
No. Friction factor and Moody chart lookup are not implemented.
Does this use pipe roughness?
No. Pipe roughness is relevant to friction factor, not to the Reynolds number calculation itself.
What are the flow regime thresholds?
Laminar: Re < 2300. Transitional: 2300 ≤ Re ≤ 4000. Turbulent: Re > 4000. These are conventional approximations for internal pipe flow.