There is considerable interest in transferring crassulacean acid metabolism (CAM) to C3 crops to improve their water-use efficiency. However, because the CAM biochemical cycle is energetically costly, it is unclear what impact this would have on yield. Using diel flux balance analysis of the CAM and C3 leaf metabolic networks, we show that energy consumption is three-fold higher in CAM at night. However, this additional cost of CAM can be entirely offset by the carbon-concentrating effect of malate decarboxylation behind closed stomata during the day. Depending on the resultant rates of the carboxylase and oxygenase activities of rubisco, the productivity of the PEPCK-CAM subtype is 74-100% of the C3 network. We conclude that CAM does not impose a significant productivity penalty and that engineering CAM into C3 crops is likely to lead to a major increase in water-use efficiency without substantially affecting yield.