With the market for products containing nanoparticles growing, improvements in the efficiency of nanoparticle synthesis are poised to have significant positive economic and environmental impacts. While many metrics have been designed for measuring the efficiency of small molecule synthesis, the use of these metrics for evaluating nanoparticle preparation has not been optimized. Here a critical evaluation of various green chemistry metrics is provided as they are applied to a common set of nanoparticle synthetic methods. The effect of the nanoparticle polydispersity on the relative greenness of different synthetic methods is also examined. Using metrics modified to account for polydispersity, a case study of gold nanoparticle syntheses is provided and three different methods of preparing monodisperse gold nanoparticles are compared. Interestingly, not all of the metrics provide the same rankings for the synthetic methods. And when polydispersity is ignored, the metrics provide a different rank order of the methods, highlighting the importance of clearly defining the desired nanoparticle size range to avoid underestimating the environmental impact.