The density of Al-rich liquid Al-Si alloys was measured contactlessly on electromagnetically levitated samples using optical dilatometry. Data were obtained for samples covering compositions up to 50 at.% Si and in a temperature range between 650 and 1500 A degrees C. The densities can be described as linear functions of temperature with negative slopes. Moreover, they increase monotonically with an increase of Si concentration. In a temperature range between 1100 and 1400 A degrees C, it can be deduced from the composition dependence of the density that virtually no excess volume arises during alloying of the pure elements. For lower temperatures an excess volume is discussed, considering the temperature dependence of Si density literature data. The density data were integrated in a thermodynamic model description of the Al-Si system. In this way volume changes during solidification and changes in phase equilibria as function of pressure can be calculated.