Reynolds Number Calculator

Variables:

• ρ = fluid density (kg/m³)
• V = mean fluid velocity (m/s)
• D = inside pipe diameter (m)
• μ = dynamic viscosity (Pa·s = kg/m·s)

Flow regime classification (pipe flow):

• Laminar — Re < 2,300: smooth, predictable, friction factor f = 64/Re
• Transition — 2,300 ≤ Re ≤ 4,000: unstable, avoid for design where possible
• Turbulent — Re > 4,000: chaotic, well-mixed, friction factor from Colebrook or Moody

Where this matters: Reynolds is the gateway number for pressure drop, heat transfer correlations, mixing assessments and erosional checks. Almost every plant fluid line is turbulent — the laminar zone shows up only in heavy-oil, viscous polymer or low-flow utility lines.

Brownfield tip: when re-rating an existing line for a new service, recompute Re. Switching from gas to liquid (or vice versa) can shift you between laminar and turbulent and break heat-transfer assumptions baked into the original design.