The slot flow meter (SFM) is a device for measuring continuously a flow of granular material. The meter is a hopper, open at the top and closed at the bottom, with vertical sides containing vertical slots. The hopper is fed continuously from above with granular material which flows out through the slots; because the flow through a slot depends upon the depth of powder upstream of the slot, the inventory of granular material in the hopper is uniquely determined by the flow rate. Hence by mounting the hopper on a load cell to measure the inventory, the flow can be inferred : the cell reading gives a continuous measure of flow rate. Experimental and theoretical work on the flow of granular material through vertical slots is described. Calibration was by an in situ technique based on the dynamic response to change of input flow. This technique was tested against direct calibration (bucket-and-stopwatch) for a pilot-scale unit installed in the particle circulation loop of a fast fluidised bed.