A system and method for weighing varying throughput of conveyed particulate material. The system has a weighing unit for measuring weight of material conveyed on a span of the conveyor and a speed monitoring unit to monitor the speed of the conveyor. A control module is programmed to calculate the instant value of material conveyed per unit length of the conveyor. An output from the control module reduces conveyor speed in response to a reduction in measured weight of material conveyed and increases conveyor speed in response to an increase in measured weight of material conveyed. High accuracy of measurement of the weight of particulate material being conveyed even when the conveyor is lightly loaded.