Method: Reynolds number for internal flow is: Re = V × D ÷ ν where V is average fluid velocity, D is pipe internal diameter and ν is kinematic viscosity. The calculator converts velocity to m/s, diameter to metres and viscosity to m²/s before calculating Reynolds number. For practical pipe-flow screening, Reynolds numbers below about 2,300 are commonly treated as laminar, values between about 2,300 and 4,000 as transitional, and values above about 4,000 as turbulent. Transition is not an exact single value in every system. Surface roughness, disturbances and flow history can affect when transition occurs.