The value of nh (τ c^h/ τ c^o -1) was defined as the dynamic hydration number, ⁿDHN, where n_h is the coordination number, and τ c^h and τ c^o are the rotational correlation times of water molecules in the cosphere and in bulk water, respectively. The correlation between the physico-chemical properties and the measured value of n_DHN for inorganic ions and biomolecules were examined. We found that the hydration properties are expressed by the linear equation of n_DHN. It was demonstrated that the hydration behavior of different homologues can be systematically discussed using universal key parameter, n_DHN.