Quantitatively evaluating progress toward energy neutral, or even energy positive, wastewater treatment necessitates reliable data on the intrinsic energy content of the wastewater. It has long been assumed that the amount of energy in wastewater is directly related to its chemical oxygen demand (COD), but the convoluted method for measuring the wastewater energy content has meant that a reliable, statistically robust relationship between COD and energy has never been drawn. In this research, we used a new drying method and analyzed a set of 107 municipal wastewater samples, with a range of COD values from 16.4 to 1151 mg/L. The results revealed a strong correlation between COD and energy content of 16.1 kJ/g of COD (p < 0.001). Reliable predictions of a wastewater's energy content can now be made on the basis of the COD measurement alone.