A seat weight sensor (10) incorporates a fluid filled bladder (12) placed in series with the load path in the seat (3), whereby a load applied to and distributed across the bladder (12) increases the pressure of the fluid therein. The pressure of the fluid is measured by a pressure sensor (20) and is substantially proportional to the magnitude of the applied load, and substantially inversely proportional to the supported area of the bladder (12). The output signal is substantially linear with respect to weight. Preferably, the amount of fluid in the bladder should be less than the capacity of the bladder when the bladder is unloaded. The seat weight sensor (10) is incorporated into an accupant restraint system (7) for controlling the safety restraint system responsive to the weight of the occupant (5).