In 2008, The Council of Australian Governments set a target to increase by 5% the proportion of Australian adults at a healthy body weight by 2017, over a 2009 baseline. Target setting is a critical component of public health policy for obesity prevention; however, there is currently no context within which to choose such targets. We analyzed the changes in current weight gain that would be required to meet Australian targets. By using transition-based multistate life tables to project obesity prevalence, we found that meeting national healthy weight targets by 2017 will require a 75% reduction in current 5-year weight gain. A reliable model of future body weight prevalence is critical to set, evaluate, and monitor national obesity targets.