For years, a simple rule of thumb has been used for predicting weight change: 3,500 calories equals one pound of body weight. However, emerging research demonstrates that it's not that straightforward, and the 3,500-calorie rule will create overly optimistic predictions of weight loss. 

This brief presents updated mathematical models that can be used to calculate the impact of calories on body weight in both adults and children. At the population level, these rules can be applied to understand the magnitude of interventions needed to close the "energy gap" that is contributing to childhood overweight and obesity.