A comprehensive water budget analysis considering the water consumed for landfill gas formation was performed for Sudokwon Landfill Site 2 (LS2) from October 2000 through December 2016. The weighted average mole ratios of carbon (C), hydrogen (H), and oxygen (O) of the total disposed waste were 30.4%, 48.8%, and 20.8% respectively. The total emitted H and O as CO2 and CH4 from landfill gas was 2,812×103 Mg, of which 28.6% (803×103 Mg) was supplied from water. The total emitted water quantity consisted of landfill gas at 8.4%, leachate at 90.2%, and vapor in landfill gas at 1.4%. The total supplied water quantity to LS2 was 22.0×106 Mg, and the quantity supplied from water included in disposed waste was 62.9%. Considering the supplied and emitted water quantity, the retained quantity in LS2 was estimated to be 12×106 Mg, and the emitted and retained quantities were 43.3% and 56.7%, respectively. Considering the retained quantity, the water content in LS2 was estimated to be 26.0%, far below the optimum level for landfill site stabilization.