The Congo Basin is of global significance for biodiversity and the water and carbon cycles. However, its freshwater availability remains highly unknown. Here, we leverage two decades of satellite and in situ observations to develop a new method that characterizes the relationship between Drainable Water Storage Anomaly (DWSA) and river discharge across the entire basin. We obtain DWSA by subtracting spaceborne estimates of the Lake Water Storage Anomaly (LWSA) and the Wetland Water Storage Anomaly (WWSA) from the Total Water Storage Anomaly (TWSA) of GRACE. We then rely on the relationship between DWSA and discharge at the outlet of a basin, which can be described as a linear, time-independent system. Such a linear relationship allows the estimation of the active water storage in the basin, or the corresponding storage point at which storage-induced discharge approaches zero. This level, in turn, operationally defines the Total Drainable Water Storage (TDWS) of a basin.