In a drilling system for drilling a borehole with drilling fluid, a flow loop communicates the drilling fluid, and a differential pressure device in the flow loop at a measurement location produces a pressure drop in flow. The measurement location can be between the borehole and a drilling choke or between a mud pump and the borehole. Piping diverts a portion of the flow at the measurement location so a flowmeter can measure the diverted flow portion. A processing unit receives a measured parameter from the flowmeter and determines a diverted flow rate of the diverted portion therefrom to correlate it to a value of the main flow rate through the flow loop at the measurement location.