Weight sensor for determining the weight of an occupant of a seat including a bladder arranged in a seat portion of the seat and including material or structure in an interior thereof which constrains fluid flow therein and one or more transducers for measuring the pressure of the fluid in the bladder. The material or structure might be open cell foam. The bladder may include one or more chambers, and if more than one chamber is formed, each chamber can be arranged at a different location in the seat portion of the seat.