Environmental agencies and utilities wishing to support environmental projects face the challenge of deciding which of the many possible projects they should support with their limited resources. Projects vary greatly in their benefits and costs, so selecting the best projects can make a major difference to the level of benefits that can be generated for a given budget. Key principles for ranking projects are presented and explained. Suitable formulas to use as a metrics for ranking projects are developed and explained. The formulas account for valuation of benefits, the effectiveness of management, time lags, behaviour change, various risks and various costs. The formulas are designed to strike a balance between theoretical rigour and reasonable simplifications. A number of common mistakes to avoid are outlined. Sample templates for project proposals and spreadsheets for ranking projects are provided, to make it easy to put the principles into practice.