ESDU 74028 gives curves and equations for use in the calculation of isentropic flow of a perfect gas; they are applicable to numerous practical situations. The curves are plotted for a range of ratios of specific heat capacities from 1 to 1.67 and for Mach numbers to unity. The equations can be applied to all Mach numbers provided no dissociation occurs. The conditions covered are 1. at a point in the flow@ the ratio of static/total temperature@ of static/total pressure@ and of velocity/speed of sound in the gas brought isentropically to rest; 2. at a point in the flow@ the ratios of kinetic/total pressure and of dynamic/kinetic pressure; 3. for the expansion of a gas through a nozzle with sonic flow at the throat@ the ratio as a function of Mach number of the pressure@ temperature and area at any point in the flow to the pressure@ temperature and area respectively at the throat; 4. in the expansion from a reservoir@ the ratio of density@ temperature and velocity to the corresponding reservoir conditions as a function of pressure ratio and 5. three mass flow functions involving total pressure and temperature@ total temperature but static pressure@ and static pressure and temperature.