Introduction: Following the removal of cysts of the jaw, the resultant bone defect is filled or left unfilled depending on its size. There is, however, no universal standard defining the volume above which a bone defect should be filled. The objective of this study was therefore to determine a threshold for bone defect filling and to assess whether this threshold value can be used for preoperative planning. Methods: We analysed cone-beam computed tomography (CBCT) and computed tomography (CT) data from 88 patients who presented with a jaw cyst. First we determined cyst volumes on the basis of CBCT and CT datasets and then we assessed the volume above which defects were filled. Results: We were able to identify a suitable threshold for bone defect filling which correlates well with the cyst diameter described in the German literature by Partsch for deciding whether to perform cyst marsupialisation or enucleation. Conclusion: The preoperative assessment of cyst volume is a simple and effective method of obtaining information about the necessity of filling bone defects and is thus a useful and reasonably accurate tool for surgical planning. Keywords: mandibular cyst, defect reconstruction, cyst enucleation, cyst volume assessment