A simple and accurate method is developed to model the effective elastic properties of early age cement pastes. It uses available data of media of which the microstructures are close to that of the predicting medium. Such technique allows us to model accurately the effect of the hydration degree on the effective elastic properties. On the other hand, the hydration degree of a hydrating cement paste is predicted from the acquisition of ultrasonic velocity. The simulated results are perfectly validated against a large variety of experimental data. This simple concept provides a great potential of application in practice.