Pressure–volume work (or PV or P - V work) occurs when the volume V of a system changes. PV work is often measured in units of litre-atmospheres where 1 L·atm = 101.325 J. 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.