People in Seoul have been actively engaged with the various activities of household waste reuse: however, how much household wastes have been reused is not well-known. This study aims to estimate the amount of reuse in Seoul. Based on a survey of 1,500 Seoul citizens, 69.5% were found to be engaged in repair, 50% in donation, 55% in purchase and 45.3% in sales. People are willing to participate in reuse activities more in the future, which will result 16.5%p ~ 25.4%p increase of citizen participation. Based on a material flow analysis of reuse organizations, resale rate versus collected goods in charity shops covered 20.6%, while the rate in the recycling centers managed by the district authorities was 61.8% and the rate in the clothing box operators, monitored by the district authorities, was 65.6%. The average annual amount of collected reused goods per reuse facility in 2019 differs according to its type: charity shops reported 135.3 tons, recycling centers collected 410 tons, and 17 tons were collected from 10,000 households by a waste clothing box operator. Through combining a citizen survey and a material flow analysis of reuse facilities, the amount of reused household waste in Seoul is estimated as 33,742 tons collected and 15,852 tons sold annually. Therefore 47% of the collected was reused and the rest were mostly moved to recycling companies. Reuse per capita per year was calculated as 1.6kg. Because ‘reuse’ or ‘preparation for reuse’ is not fully covered in official waste statistics, the estimated result of this study adds up household waste generation by 0.4%, which contributed to the increase of recycling rate by 0.1%p. The quantification of reuse is essential to achieve Seoul’s policy goals to become a circular city. So three basic conditions are recommended : 1) to single out the amount of ‘preparation for reuse’ excluding customer-to-customer (C2C) transactions; 2) to figure out the exact amount of reused goods sold and; 3) to establish a data collection and monitoring system covering the amount of collected and traded goods in reuse facilities managed or monitored by the district authorities.