In 2022, British International Investment (BII), the 8.1 billion pound development finance arm of the British government, implemented a new impact measurement system called the Impact Score to align its financial and impact objectives systematically. The tool was designed to measure and manage impact across three dimensions, including productivity, sustainability, and inclusion, and to guide portfolio-level decision-making. By integrating the Impact Score into its investment framework, BII aimed to achieve its dual mandate of achieving development impact while maintaining moderate financial returns. The case examines BII's efforts to balance financial and impact trade-offs through portfolio construction and features a simulation exercise to explore the practical application of the Impact Score in optimizing impact and financial outcomes.