This paper sets out and applies a new methodology for determining whether or not economic activity is environmentally sustainable. By comparing current environmental impacts with standards of environmental sustainability, it calculates the ‘sustainability gaps’ (SGAPs) with respect to different environmental impacts. For the UK, SGAPs are computed for CO2, SO2, and other air pollutants. Across none of these environmental themes can current UK use of the environment be said to be sustainable. The SGAP indicators can be combined with current trends to show how long it would take, on continuation of the trends, for the sustainability standard to be attained. The paper calls this indicator the ‘Years to Sustainability’ (YS) measure. For the policy objective of sustainable development to be made fully operational, it is necessary for the concept of environmental sustainability to be clearly defined by quantitative indicators. On the basis of the indicators set out in this paper it can be judged whether economic activity is moving towards or away from environmental sustainability, and at what speed.