The function DUP/µ used in fluid flow calculations to estimate whether flow through a pipe or conduit is streamline or turbulent in nature. D is the inside pipe diameter, U is the average velocity of flow, P is density, and µ is the viscosity of the fluid. Different systems of units give identical values of the Reynold's number, and values much below 2100 correspond to streamline flow, while values above 3000 correspond to turbulent flow.