In equations, the function:

$\Delta H = \Delta U + P \Delta V \,$

is called enthalpy change, or change in the heat content of the system, and is internal energy change ΔU less the pressure volume work energy change PΔV of a system’s enthalpy on going from an initial state to a final stated. This difference is quantified by the expression:

$\Delta H = H_f - H_i \,$

or the final system state measure of enthalpy Hf less the initial system state measure of enthalpy Hi.