In one example, a system for loading a build material powder supply container for 3D printing includes a dispenser to dispense a build material powder into a supply container, a device to measure a density of the build material powder in the supply container, a compactor to compact the build material powder in the supply container, and a controller operatively connected to the measuring device and the compactor. The controller is programmed to control the compactor to compact the build material powder in the supply container until a measured density reaches a threshold density.