The IAPWS-95 formulation explicit in Helmholtz free energy proposed by Wagner and Pruß (The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use, Journal of Physical and Chemical Reference Data 2002 31(2), 387–535) is extended to calculate the volumetric property of the fluid water from 1 GPa and 273 K to 20 GPa and 4273 K. Comparison with large experimental and molecular dynamic simulation data above 1 GPa shows that the equation of state can reproduce the volume of the fluid water with an average absolute deviation of 0.52%. Thus the original IAPWS-95 formulation together with the extended part can be used in a much larger temperature–pressure region: 273–4273 K and 0–20 GPa. In addition, this paper also reports a reliable and highly efficient method to calculate the saturated properties of water so that the equation of state can be conveniently applied in the study of fluid inclusion: calculating homogenization pressures, homogenization densities (or molar volumes) and isochores. Computer code of the model can be obtained from the first author.