d = 0.001×Mw / Vm; where Mw is the molar mass in grams and Vm is the molar volume in m3/mol.
Z = 1000×p×Vm/(R×T) = 1 + B/Vm + C/Vm2,
where p is in kPa, B = åbi+1/T i, C = åci+1/T i, and R is the gas constant. The summation begins with i = 0.