Practical lithium metal batteries have been researched worldwide, but due to excessive “Li reservoir” in the anode, quantification of the authentic reversibility of practical cells remains unresolved. Quantitative method for assessing the reversibility and irreversibility of Li anode is thereby essentially needed. Here we propose an index system composed of several quantitative parameters to evaluate the reversibility of practical lithium metal batteries. Mathematical relationships between these parameters and cycle number are successfully established based on the mass of active and inactive Li in the cycled Li anode. By establishing a novel and universal analytical method, mass of active Li can be quantitatively distinguished from the inactive one of cycled lithium metal anode in Ah-level pouch cells. Through fitting quantified data of active and inactive Li under different cycles, values of parameters representing for the reversibility and irreversibility of Li are quantitatively decoupled. The quantitative analysis method presented in this work provides new criteria to accurately assess practical lithium metal batteries for more reliable degradation analysis and failure prediction.