In processes such as fluid catalytic cracking, combustion of coal, calcination of lime, gas and particles are brought into contact at high gas velocities with particle entrainment. To sustain gas solid contact in such beds, known as risers, it is essential to introduce particles along with the gas at the bottom of the bed. Modeling the performance of these vital processes can help in their rational design and further development. Quantitative description of nature of gas solid contact is necessary to develop performance models. Particle holdup as a function of gas and particle flow rates is needed for such models. It is observed that slip velocities are higher than the terminal velocity of particles. This is generally attributed to aggregation of particles to move together as clusters while gas aggregates to flow as voids/bubbles. Clusters and voids form, coalesce and break and are in a state of dynamic equilibrium. A model is proposed to estimate size of clusters considering that “ratio of cluster volume to void volume is equal to the ratio of cluster to void volumetric fractions in the bed”. The model predictions compare well with the literature experimental data. Slip velocities estimated from the size of clusters explain the reported experimental observations on gas solid as well as liquid solid flows in risers.