The use of leftover bath water for household laundry is a practice unique to Japan. This study investigated the annual reduction in the environmental load that household laundry achieved using leftover bath water and ultra-concentrated liquid detergents, based on calculated GHG (greenhouse gas) emissions and life cycle cost assessments from actual measurements of model household laundry conditions, while taking into account differences in washing machines and detergents. Compared to conventional laundry practices (pulsator-type washing machine, laundry volume of 2.5 kg/wash, powder detergent), the results showed it was possible to reduce GHG emissions by 22％ by switching to a drum-type washing machine and by 24％ by using leftover bath water. The use of high-temperature leftover bath water was also found to give similar detergency between winter and spring-fall. The parameters of the laundry practice generating the least GHG emission volumes were the use of leftover bath water, drum-type washing machine, ultra-concentrated liquid detergent, and bulk washing cycles. Compared to conventional laundry practice, this resulted in a 59％ reduction in GHG emissions from the washing process and a 59％ reduction in cost. Due to sanitary and safety considerations, the volume of detergent cannot be reduced when using leftover bath water. However, the volume of water used can be halved. The study results confirm that this laundry method can significantly reduce GHG emissions and cost while maintaining detergency levels.