To estimate the energy gap (i.e. degree of positive energy balance) that is causing weight gain in the population of China in order to provide a quantitative goal for how much behaviour change is required to prevent excess weight gain in the population. This is based on data collected in the China Health and Nutrition Survey. 2288 adults (20–45 years) who took part in both the 1989 and 2000 surveys were selected. Results: from 1989 to 2000, the average weight of this population increased from 55.4 kg to 59.1 kg. The medium increasing rate was 0.33 kg year−1. During this time, the prevalence of overweight in the population increased dramatically from 9.0% to 23.2%. By assuming that each kilogram of body weight gained represents 7700 kcal, the estimated medium energy accumulation in the sample was 7.0 kcal day−1 and the 90th percentile was 22.5 kcal d−1. Further, assuming that the energy derived from mixed composition diets is stored with an average efficiency of 50%, the estimated energy gap for the population was 45 kcal day−1. This is the degree of positive energy balance that is causing weight gain in 90% of the population. The estimate suggests that the behaviour change needed to close the energy gap is small and achievable without dramatically altering current lifestyle in China. For example, reducing energy intake by just 2–3% less each day or walking an extra 10–15 minutes each day could offset weight gain in roughly 90% of the population in China. By providing an estimate of the energy gap it can be predicted that weight gain in most of the population of China could be prevented with small lifestyle changes. It is critical that these lifestyle changes be implemented in order to prevent acceleration of the obesity epidemic in China.