The deep inferior epigastric perforator (DIEP) flap is used with increasing frequency in post-mastectomy breast reconstruction. Preoperative mapping with CT angiography (CTa) is crucial in reducing surgical complications and optimizing surgical techniques. Our study’s goal was to investigate the accuracy of conventional CT (cCT), performed during disease staging, compared to CTa in preoperative DIEP flap planning. In this retrospective, single-center study, we enrolled patients scheduled for mastectomy and DIEP flap breast reconstruction, subjected to cCT within 24 months after CTa. We included 35 patients in the study. cCT accuracy was 95% (CI 0.80–0.98) in assessing the three largest perforators, 100% (CI 0.89–100) in assessing the dominant perforator, 93% (CI 0.71–0.94) in assessing the perforator intramuscular course, and 90.6% (CI 0.79–0.98) in assessing superficial venous communications. Superficial inferior epigastric artery (SIEA) caliber was recognized in 90% of cases (CI 0.84–0.99), with an excellent assessment of superficial inferior epigastric vein (SIEV) integrity (96% of cases, CI 0.84–0.99), and a lower accuracy in the evaluation of deep inferior epigastric artery (DIEA) branching type (85% of cases, CI 0.69–0.93). The mean X-ray dose spared would have been 788 ± 255 mGy/cm. Our study shows that cCT is as accurate as CTa in DIEP flap surgery planning.