Cohesive granular materials play a major role in nature and industry. Cohesive interactions between particles have various physico-chemical origins such as capillary bonding, Van der Waals forces, and a cementing matrix. Despite extensive experimental and numerical work on these materials, their mechanical behavior under the action of external loading is still poorly understood. In this work, we use the Discrete Element Method (DEM) to perform an extensive parametric study of isotropic compaction of cohesive granular materials in which the cohesive action between two particles is modeled as a constant adhesion force. We propose a compaction law expressing the relative void ratio as a generic function of a new dimensionless number using parameters such as the cohesive force, the normal stiffness and the isotropic pressure. We also show that the finite size and wall effects can lead to deviations from this law as a result of stress concentration at the boundary of the system.