Precipitation calculations for Mg alloys have been performed using an in-house precipitation model. The size distribution, number density and volume fraction of precipitates are complex functions of thermodynamics, nucleation, and diffusion kinetics dependent on solute composition and heat treatment parameters. The mathematical description and the numerical approach for the precipitation model are given in details. The precipitation model is dynamically linked to the FTlite light alloy database of the FactSage system for accurate input of thermodynamic data. The simulation results are compared with the available experimental data for Mg alloys such as Mg-Al, Mg-Zn and Mg-Al-Zn alloys to ascertain the predictive ability of the precipitate model. The precipitation model is also applied to deduce the precipitation kinetics in AZ91 + Ca alloy in comparison to conventional AZ91 alloy.