Uncertainty in the experimentally-derived IAPWS95 properties are very small. Specifically, the mean square relative error in enthalpy, entropy and Gibbs free energy for IAPWS95 equation is less than ∼ 10 -9 . The uncertainty in the ideal gas part of the property, derived analytically, does not contribute to the total uncertainty. Thus the total uncertainty in experimental properties comes from the residual part, and is as we note, it is very small.