Bangladesh is a severely flood-affected country that ranks fifth globally. Although it is well established that regional and community-based flood early warning systems (FEWS) may minimize the effects of floods, cost-benefit analyses of FEWS are still limited in Bangladesh. This is the first study to assess the cost-benefit of the FEWS in lower Brahmaputra River basin in Bangladesh which included 1000 household surveys, 32 focus group discussion, and key informant interviews. The findings indicated that during the floods, families considered the FEWS to be useful and trustworthy, enabling them to preserve household assets, agricultural, and livestock expenditures worth of BDT 267713 (USD 2525.59) per household. Based on the different scenarios, the benefit-cost ratio (BCR) ranged from 79.87 to 213. About 92% of the respondents expressed willingness to pay an annual charge of BDT 100.63 (USD 0.94) for five years if the community disaster committees were to handle the current FEWS. This might generate BDT 100637 (USD 949.40) annually, to pay for the system's maintenance and operations. As communities' advance confidence in the FEWS system and lead times are enhanced, FEWS increasingly changes their behaviors over time, leading to enhanced social capital and a broader range of early interventions that lessen preventable loss and damage. By extending the forecast lead time by two days, the present savings could increase by 21.7 times. To increase funding, government and non-government organizations may make decisions based on the findings of the cost-benefit analysis. This study also proposed a FEWS applicable in the local level.