A fast technique for simulating fluid pockets enclosed in an elastic body has been developed using the Finite Element Method. By treating fluid pressure as a force boundary condition, the relationship between the volume and pressure of a fluid cavity can be enforced with an iterative solver. This computational approach has been shown to agree with experimental data taken from a gelatin phantom that contains a small fluid pocket. Combining linear methods and condensation techniques with this iterative solver, fast simulation of elastic bodies that include fluid pockets can be achieved. For example, an extension of a two dimensional needle insertion simulation can be carried out at 512Hz for a 24 node incompressible fluid pocket.