In an example implementation, a powder metering device to move powder in a 3D printing system includes a rotor comprising a central hub and peripheral wheel to rotate about a shaft. Compartments are formed between the hub and the wheel by a plurality of fins that emanate from the hub at first fin ends and affix to the wheel at second fin ends. A top and bottom disk each have a planar surface to lie flush against and to seal, respectively, top and bottom perimeters of each compartment as the compartments rotate about the shaft between a powder receiving position and a powder dispensing position.