The flowrate of a stream discharged from a drain in a vessel is measured by imaging the stream, measuring a diameter of the stream image, and calculating the flowrate based on the measured diameter. A video camera may be aimed below the drain for imaging the stream. A computer is joined to the camera for measuring the diameter of the stream and calculating a corresponding flowrate therefor. Either the pressure head above the drain or the profile of the stream below the drain may be used in calculating the discharge stream flowrate