Mooney-Rivlin material parameters are estimated for an isotropic, homogeneous one element square rubber membrane undergoing biaxial stretch. The deformed geometry and force files are first created by solving a boundary value problem (specified displacements). The parameter estimation here uses a linear least-squares approach to miminimise the force residuals. The parameter estimated is the c1 Mooney-Rivlin param for each of the 4 nodes.