Reynolds number is a pure number which determines the type of flow of a liquid through a pipe. It is denoted by N_{R}.

It is given by the formula: **N _{R} = (V_{c} ρD) /η**

Where V_{c} is the critical velocity. ρ is the density. n is the co-efficient of viscosity of the liquid and D is the diameter of the pipe.

If N_{R} lies between 0 and 2000, the flow of a liquid is said to be streamline. If the value of Nit is above 3000, the flow is turbulent. If N_{R} lies between 2000 and 3000, the flow is neither streamline nor turbulent, it may switch over from one type to another.

Narrow tubes and highly viscous liquids tend to promote stream line motion while wider tubes and liquids of low viscosity lead to turbulence.