BACKGROUND: Reverse shoulder arthroplasty requires a re-tensioning of the deltoid to obtain active elevation and implant stability. Currently, there is no objective and reliable technique described for the preoperative planning of reverse shoulder prosthesis or the postoperative evaluation of deltoid tension and arm lengthening. The purpose of this investigation was to outline a standardized technique for measuring deltoid length and to preoperatively plan a reverse shoulder arthroplasty, and to determine whether complications are related to inadequate deltoid lengthening. METHODS: Fifty-eight patients were included in this radiographic review. Variations in humeral length, overall arm length, and the height of the subacromial space were evaluated before and after reverse shoulder arthroplasty. RESULTS: The average postoperative lengthening of the humerus was 2 +/- 7 mm (range, -9-16, P = .243) and the arm was lengthened 23 +/- 12 mm (range, 1-47, P <.001). Measured preoperative and postoperative differences of the subacromial space were statistically significant when comparing the operated and contralateral arm (P < .0001). Lengthening was not correlated to sex (P = .242), acromial fractures, or neurological complications (P = .83). However, in cases of postoperative instability, both humeral and overall arm lengthening were statistically lower (P < .0001). CONCLUSION: A technique to preoperatively plan adequate deltoid tensioning using radiographs of the contralateral arm is described. This technique is critical in challenging cases and postoperatively in cases of complication to assess the deltoid length. Subjective intraoperative criteria to evaluate deltoid tension should be replaced by objective measures to prevent insufficient or excessive deltoid tension. LEVEL OF EVIDENCE: Level 3.