Pressure–volume work (or PV or P-V work) occurs when the volume  of a system changes. PV work is often measured in units of litre-atmospheres where . However, the litre-atmosphere is not a recognized unit in the SI system of units, which measures P in pascals (Pa), V in m, and PV in joules (J), where 1 J = 1 Pa·m. PV work is an important topic in chemical thermodynamics.
For a process in a closed system, occurring slowly enough for accurate definition of the pressure on the inside of the systems wall that moves and transmits force to the surroundings, described as quasi-static', work is represented by the following equation between differentials:
where
* (inexact differential) denotes an infinitesimal increment of work done by the system, transferring energy to the surroundings;
* denotes the pressure inside the system, that it exerts on the moving wall that transmits force to the surroundings. In the alternative sign convention the right hand side has a negative sign.
* (exact differential) denotes an infinitesimal increment of the volume of the system.
Moreover,
where  denotes the work done by the system during the whole of the reversible process.
The first law of thermodynamics can then be expressed as
(In the alternative sign convention where W = work done on the system, . However,  is unchanged.)