Lake Ranau has experienced increasing environmental pressure due to population growth, tourism development, agricultural activities, and floating net cage (FNC) aquaculture within its watershed. Previous studies reported that the lake is moderately polluted, with phosphate concentrations exceeding the Class I water quality standard (Wiata, 2024), while eutrophication, β-mesosaprobic conditions, algal blooms, hypoxia, and periodic fish mortality have also been observed (Efendia et al., 2023; Syahfitri et al., 2025). However, studies integrating water availability, water demand, water balance, and water quality remain limited. Therefore, this study aimed to analyze water availability, water demand, water balance, and water quality as the basis for developing sustainable water quality management strategies for Lake Ranau. A quantitative descriptive-analytical approach was employed using secondary data collected during 2016–2025. Water availability was estimated using the monthly 90% dependable discharge (Q90) based on the Weibull method, while water demand was calculated for the domestic, non-domestic, and irrigation sectors. Water quality was evaluated using 12 physical, chemical, and biological parameters through the STORET and Pollution Index (PI) methods in accordance with Government Regulation No. 22 of 2021 and the Indonesian Ministry of Environment Decree No. 115 of 2003. The results showed that the Q90 dependable discharge ranged from 4.72 to 10.72 m³/s, while total water demand ranged from 0.041 to 0.328 m³/s, resulting in a surplus water balance throughout 2025. Although most water quality parameters complied with the Class I water quality standard, phosphate, COD, BOD, DO, nitrite, and TSS exceeded the permissible limits in specific zones and seasons. Both the STORET and PI methods consistently classified Lake Ranau as moderately polluted, with Zone II (Banding Agung) experiencing the highest pollution pressure. These findings indicate that Lake Ranau currently has sufficient water availability to support existing water uses, whereas water quality remains the primary management concern. Therefore, sustainable management should prioritize nutrient input reduction, particularly phosphate, sustainable management of floating net cage aquaculture, improvement of domestic wastewater treatment, runoff control within the catchment area, and continuous water quality monitoring. The novelty of this study lies in integrating water quantity and water quality assessments within a single analytical framework to support evidence-based and sustainable lake management.