A knowledge of the degree of expansion of fluidized beds is needed by designers in order to calculate conversions in fluidized-bed chemical reactors and, in combustors or incinerators, the relative areas of heat-transfer surfaces immersed in the bed and the freeboard. Because differential and absolute pressures are relatively easy to measure in fluidized beds even at high temperatures, they can provide a means for measuring and controlling conditions in the bed. In this paper, protocols are presented for (a) predicting bed expansion and density and (b) using pressure measurements to deduce hydrodynamic conditions in the bed, notably bubble velocities, which determine the gas residence time. To test the validity of the theory presented, experiments have been done in two columns, one of which was 300 mm internal diameter using four sands at low gas velocities and the other in which a fluidized catalytic cracking catalyst and hydrated alumina were fluidized at velocities up to and beyond the velocity at which transition to turbulent fluidization occurs in a column of 290 mm internal diameter. The rise velocities deduced are alarmingly high compared with values predicted from correlations in current widespread use.