The flux of diffusing atoms, J, is used to quantify how fast diffusion occurs. The flux is defined as either the number of atoms diffusing through unit area per unit time (atoms/m2-second) or the mass of atoms diffusing through unit area per unit time, (kg/m2- second).
For example, for the mass flux we can write
J = M / At ≅ (1/A) (dM/dt) (Kg m-2 s-1)
where M is the mass of atoms diffusing through the area A during time t.