A method for identifying vortical structures and calculating the circulation of the wake trailed from a rotor blade is introduced, similar to a Trefftz plane analysis for fixed-wings. A depth-first search algorithm was developed based on a vorticity threshold to identify clusters of cells defining a single vortex based. This algorithm is computationally efficient and is used to identify both positive and negative circulation. Furthermore, a non-dimensional vorticity threshold parameter was introduced that was a function of an assumed vortex model (the n = 2 Vatistas model in this study) and the blade loading coefficient of the rotor. The developed algorithm was applied to high-resolution flow field images behind a rotor hovering out-of-ground effect obtained using particle image velocimetry measurements. These measurements were used to validate and show potential applications of this method. The net circulation trailed by the blade was used to infer the blade bound circulation and compared to predictions from blade element momentum theory (BEMT). The calculated wake circulation was found to be more sensitive to threshold selection than the tip-vortex, resulting in the calculated bound circulation being sensitive to threshold selection. © Copyright 2018 by AHS International, Inc. All rights reserved.