Reynolds Number Calculator

Formula

• Re = V × D / ν
• ν entered as ×10⁻⁶ m²/s (kinematic viscosity)
• Pipe: laminar Re < 2300, transitional 2300–4000, turbulent > 4000

About this calculator

Part of our Thermal & Fluids Calculators collection. Heat transfer, pipe flow, Reynolds number, and thermodynamic cycles.

How it works

Reynolds number equals velocity times characteristic length divided by kinematic viscosity. Pipe flow typically transitions near 2300. Flat plate and sphere modes use different threshold values. Use this to pick friction factor models and heat transfer correlations.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

Water at 20°C in 50 mm pipe at 2 m/s (ν = 1.004 ×10⁻⁶ m²/s).

1. Re = 2 × 0.05 / (1.004×10⁻⁶)
2. Re ≈ 99 600
3. Well above 4000 → turbulent flow

Result: Re ≈ 99 600, turbulent pipe flow.

When to use

• Classifying internal pipe flow before picking friction factor
• Checking whether laminar or turbulent heat transfer correlations apply
• Flat plate or sphere external flow screening

Limitations

• Kinematic viscosity input uses ×10⁻⁶ m²/s scale
• Transition limits are approximate; exact Re depends on disturbances
• Does not compute friction factor or pressure drop directly

FAQ

Why is viscosity in ×10⁻⁶?

Water at room temperature has ν ≈ 1×10⁻⁶ m²/s. Enter 1.004 for water, 1.5 for light oil, etc.

What about flat plate flow?

External flat plate mode uses Re_c ≈ 5×10⁵ as the laminar-to-turbulent boundary for the correlation context.