Hydrocarbon depletion and fluid injection cause compaction and stretching of the reservoir and overburden layers. 4D pre-stack seismic data can be used to detect these changes because compaction/stretching causes changes in traveltimes and seismic velocities. We show that, by using two different petro-elastic models at varying effective pressures, a good approximation is to assume that the fractional changes in layer thickness and seismic velocity, are related by a linear function of the layer thickness. This function (dilation factor) is negative and its absolute value generally decreases (shale, low porosity) or increases (sandstone, high porosity) with increasing layer thickness and decreasing effective pressure. The dilation factor, which can be calculated from time-lapse seismic data, can be used to predict reservoir compaction/stretching as a function of depth and surface subsidence.