In physics and engineering, mass flux is the rate of mass flow per unit of area. Its SI units are kg m s. 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. 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.