This paper analyses the Family Planning Programme (KB) in Indonesia through the perspective of Economic Analysis of Law (EAH) developed by Richard A. Posner, with a focus on legal efficiency in internalising demographic externalities and maximising social wealth. This study uses a legal-economic approach, combining normative legal research methods with economic analysis of public policy, particularly Law No. 52 of 2009 on Population Development and Family Development, Government Regulation No. 87 of 2014 concerning Population Development and Family Development, Family Planning and Family Information Systems, and Presidential Regulation No. 72 of 2021 on Accelerating Stunting Reduction. The results show that family planning policies are legal interventions that meet the Kaldor–Hicks efficiency criteria, as their social benefits, such as a decrease in the total fertility rate (TFR), increased female economic participation, and the creation of a demographic bonus, aggregate to exceed their implementation costs. However, inefficiencies still occur due to increased transaction costs and unmet needs, which reflect market failures and bureaucratic constraints. The analysis shows that the 2025 fiscal efficiency policy, which cuts the budget for contraceptive procurement, has the potential to cause greater social deadweight loss in the future. Therefore, it is recommended that family planning policies be directed towards long-term efficiency through budget protection based on incremental cost-effectiveness ratio (ICER), reduction of transaction costs, and strengthening of access rights to family planning services.