We conducted a comprehensive overall tap water hardness assessment for Japan. Tap water was collected from 665 points throughout Japan, and its standing position was quantitatively clarified by prefecture. The mean and median hardness of tap water in Japan was 48.9 ± 25.8 (1σ SD) and 46.0 mg/L, respectively. Compared with 27 other countries, Japan exhibited soft water with low-mineral content. Water hardness tended to be high in the Kanto region and low in the Hokkaido and Tohoku regions. The impact of the distribution system's water pipes on tap water hardness is discussed using a unified index to evaluate variations in hardness from raw to tap water. A comparison of the variations in hardness showed that hardness variations from raw to purified water and from purified to tap water exhibited a 20% variation range. Furthermore, tap water hardness and its fluctuations in any region of Japan were found to be caused by raw water hardness. It was demonstrated that the distribution pipe system had no large impacts on water hardness.