In physics and engineering, mass flux is the rate of mass flow per unit of area. Its SI unit is kg⋅s⁻¹⋅m⁻². The common symbols are j, J, q, Q, φ, or Φ (Greek lowercase or capital phi), sometimes with subscript m to indicate mass is the flowing quantity.

This flux quantity is also known simply as "mass flow". "Mass flux" can also refer to an alternate form of flux in Fick's law that includes the molecular mass, or in Darcy's law that includes the mass density. Less commonly, the defining equation for mass flux in this article is used interchangeably with the defining equation in mass flow rate.

Mass flux is defined as the limit $${\displaystyle j_{\text{m}}=\lim _{A\to 0}{\frac {I_{\text{m}}}{A}},}$$ where $${\displaystyle I_{\text{m}}=\lim _{\Delta t\to 0}{\frac {\Delta m}{\Delta t}}={\frac {dm}{dt}}}$$ is the mass current (flow of mass m per unit time t) and A is the area through which the mass flows.

For mass flux as a vector j_m, the surface integral of it over a surface S, followed by an integral over the time duration t₁ to t₂, gives the total amount of mass flowing through the surface in that time (t₂ − t₁): $${\displaystyle \Delta m=\int _{t_{1}}^{t_{2}}\iint _{S}\mathbf {j} _{\text{m}}\cdot \mathbf {\hat {n}} \,dA\,dt.}$$

The area required to calculate the flux is real or imaginary, flat or curved, either as a cross-sectional area or a surface.

For example, for substances passing through a filter or a membrane, the real surface is the (generally curved) surface area of the filter, macroscopically – ignoring the area spanned by the holes in the filter/membrane. The spaces would be cross-sectional areas. For liquids passing through a pipe, the area is the cross-section of the pipe, at the section considered.

The vector area is a combination of the magnitude of the area through which the mass passes through, A, and a unit vector normal to the area, ${\displaystyle \mathbf {\hat {n}} }$. The relation is ${\displaystyle \mathbf {A} =A\mathbf {\hat {n}} }$.

If the mass flux j_m passes through the area at an angle θ to the area normal ${\displaystyle \mathbf {\hat {n}} }$, then $${\displaystyle \mathbf {j} _{\text{m}}\cdot \mathbf {\hat {n}} =j_{\text{m}}\cos \theta }$$ where · is the dot product of the unit vectors. That is, the component of mass flux passing through the surface (i.e. normal to it) is j_m cos θ. While the component of mass flux passing tangential to the area is given by j_m sin θ, there is no mass flux actually passing through the area in the tangential direction. The only component of mass flux passing normal to the area is the cosine component.