The specific enthalpy of a uniform system is defined as h = ⁠ H / m ⁠ where m is the mass of the system. The SI unit for specific enthalpy is joule per kilogram. It can be expressed in other specific quantities by h = u + p v , where u is the specific internal energy, p is the pressure, and v is specific volume, which is equal to ⁠ 1 / ρ ⁠, where ρ is the density.