Abstract: The flow regime zone which the flow belongs to can not be predetermined for the equations given by the Reynolds number for the second and third type of problems (flow rate, Q, and pipe diameter, D are un-known)of hydraulic calculation, so that the trial and iteration have to be needed. On the basis of the limitation of turbulent zone recommended by the industry, the equations are derived or modified, the least square method and some constants are fitted, the equations obtained can be used to predetermine the flow regime zone for all the three problems of hydraulic calculation, the trial can be avoided.