The pressure acting on the surface at x can be calculated from the rate of change of momentum of molecules normal to the surface. For incoming molecules the molecules from x′ give the following contribution to the pressure on the surface:

Substituting for ρ (θ) and integrating gives the total contribution of incoming molecules to the pressure, pin (SI unit: Pa), at x:

From Equation 3-15 it is shown that:

The computed incoming pressure contribution is available on surfaces as fmf.Pin.

The surface is also emitting molecules at a rate J (in most cases J = G, but in general this does not need to be the case — for example, the surface can be outgassing in addition to emitting the adsorbed molecules so that J = G + J out). The emitted molecules also have a normal momentum change that contributes to the pressure in the following manner:

Thus the total pressure contribution from emitted molecules, pout (SI unit: Pa), is:

Substituting for the mean velocity (using Equation 3-18 with ) the equation obtained:

where the same result holds in 2D and 3D. The outgoing pressure contribution is available in the Free Molecular Flow interface as fmf.Pout.

Finally the total pressure acting on the surface, p (SI unit: Pa), is given by:

This is computed in the physics interface as the dependent variable p.