where v is the velocity, ρ the density, S the stress tensor, ε the strain tensor, C is the elasticity tensor (or stiffness tensor), and Fv is a possible body force. The equations are valid for both isotropic and anisotropic material data.

The unknowns solved for are three components of the velocity vector and six components of the strain tensor in 3D. The out-of-plane components of the velocity and the strain are not computed per default in 2D. This can be changed by selecting Include out-of-plane components check box. It can be relevant in some cases, for example for anisotropic materials, where the out-of-plane velocity may become different from zero for in-plane loads. This formulation is often referred to as 2.5D.