Volumetric strains during triaxial tests of 44 sandstone materials were analysed in order to understand the physics of dilation. The samples originate from different North-Sea fields, and the porosities vary from 1 to 39 percent. Different confining stresses were applied to the same material under dry and saturated conditions and the results analysed. The results showed no clear boundary between dilation dominated and compaction dominated behaviours with respect to porosity or confining stress. However, two regimes could be identified: one with low porosities where dilation occur, and one with high porosities where dilation do not occur. The critical porosity separating the two regimes was found to depend on the confining stress. The analysis also showed that the shear stress level at the onset of dilation depends on the ratio between the in situ horizontal stress under which the sample was buried, and the confining stress. A mathematical formula relating the onset of dilation to porosity, confining stress and in situ horizontal stress has been established. The dependency of in situ horizontal stress is likely to be ascribed to core damage. This assumption is supported by previous experimental work on artificial sandstone. Volumetric strain analysis showed that fully saturated samples dilate more than corresponding dry materials. The onset of dilatancy for both saturated and dry materials can be described by the same mathematical formula, however with different coefficients. The results from the use of the model showed that the onset of dilation could be estimated with a fair accuracy if the porosity and the in situ horizontal stress are known.