The challenge: Global human population is expected to continue to grow over the next four decades. The ensuing demands for water, food and energy are expected to intensify land-use conflicts, contribute to greenhouse-gas emissions, and exacerbate threats to natural ecosystems and wildlife. These conflicts will be most severe in the developing tropics where population growth has been most rapid, the people are poorest, and biodiversity is richest and yet most threatened globally. Decision makers in the developing tropics, therefore, are faced with having to balance human development on the one hand and environmental protection on the other. The Land Use Calculator is a decision-support tool developed by ecologist, Lian Pin Koh. It is primarily targeted at land-use decision-makers in the tropics. Drawing data from the World Bank and other international agencies, this tool allows end-users to evaluate the implications and tradeoffs of pursuing alternative development scenarios by simultaneously accounting for the societal priorities of agricultural production, economic development, carbon conservation and biodiversity protection. The Land Use Calculator is designed to be intuitively user-friendly. Users simply have to specify a few environmental and socioeconomic parameters describing a landscape scenario. And the tool determines the implications of that scenario in terms of biodiversity, carbon stocks, greenhouse-gas emissions, financial returns of the land and even employment opportunities. The Land Use Calculator is a tool for rapid scenario analysis of land-use implications. The tool is useful not only for decision-makers to address real-world challenges, but also for classroom teaching of topics including conservation biology, sustainable development, environmental economics and global change biology.