Abstract: Different size distributions determine different packing densities. A formula of packing density with continuous grain size distribution for cement pastes was developed. Based on measuring the water requirements of the cement-slag powder system with different size distributions in the same fluidity degree，the packing density of the different size distributions of the slag and their water requirements were calculated by the formula，and the results agree well with the testing ones. The difference among the water demands of 15 samples used in the experiment is 4% to 10%. In addition，the validity of the method was verified through experiment. The formula could simulate the packing density of the blend materials such as cement，fine-slag，and fly ashes.