Lithium metal negative electrodes provide a pathway to high specific energy density electrochemical energy storage, particularly attractive for use in electric vehicles. One significant limitation to the implementation of Li negative electrodes is Coulombic inefficiency, namely the loss of capacity to irreversible processes. Multiple degradation pathways, which include electrolyte reduction, lithium corrosion, and the formation of dead lithium, contribute to these losses, but few methods of characterization can distinguish the various modes of degradation. Here, a method of quantifying the amount of lithium metal in the electrode via operando synchrotron X-ray diffraction (XRD) is developed. Through this method, how much lithium actively cycles and how much is lost to the three degradation mechanisms noted above can be differentiated. Additionally, the operando synchrotron XRD provides a unique view of lithium metal corrosion, whereby the corrosion current of electrodeposited lithium metal can be measured, which reveals that lithium corrodes when electrically connected to the copper current collector as well as in the form of dead Li metal. Observing the corrosion of both electrically connected and disconnected lithium provides new insights into corrosion mechanisms in lithium metal batteries. This approach addresses the lack of quantification methods for capacity losses and provides a more complete understanding of Li corrosion, both of which can aid in the design of long-lasting Li metal negative electrodes.