Quantity and quality of raw domestic wastewater are reviewed and discussed for packaged plants, known as johkasou, in Japan. A method for the estimation of johkasou capacity in terms of population equivalent, or PE, was introduced and discussed in this paper. It was found that the pollutant loads of domestic wastewater were respectively BOD 45 g, CODMn 23 g, SS 37 g, TN 9.3 g and TP 1.1 g per capita per day on average. It shows that the BOD load of wastewater from kitchens is higher than that of black water. The estimation of the johkasou capacity needed for buildings can be done using a series of formulas, which forms an important estimation method for the implementation of on-site wastewater treatment.