This International Standard specifies methods for the calculation of compression factors of natural gases, natural gases containing a synthetic admixture and similar mixtures at conditions under which the mixture can exist only as a gas. This part of ISO 12213 specifies a method for the calculation of compression factors when the detailed composition of the gas by mole fractions is known, together with the relevant pressures and temperatures. The method is applicable to pipeline quality gases within the ranges of pressure p and temperature T at which transmission and distribution operations normally take place, with an uncertainty of about ±0.1 %. It can be applied, with greater uncertainty, to wider ranges of gas composition, pressure and temperature (see annex E). More detail concerning the scope and field of application of the method is given in part 1 of this International Standard.