Environmental agencies and organisations face the challenge of deciding which of the many possible environmental projects they should support with their limited resources. Projects vary greatly in environmental benefits and costs, so selecting the best projects can make a major difference to the level of environmental benefits that can be generated for a given budget. Key principles for ranking environmental projects are presented and explained. A suitable formula to use as a metric for ranking projects is developed and explained. The formula accounts for environmental values, the effectiveness of management, time lags, behaviour change, various risks and various costs. The suggested formula includes a number of appropriate simplifications. A number of common mistakes to avoid are outlined.