Background: Determining the correct size (volume) of the prostate is imperative for patients with prostate cancer. Prostate size can affect many parameters in patient care such as cores in a biopsy taken and available treatment options. Traditionally, most patients underwent transrectal ultrasound (TRUS) during prostate biopsy to estimate prostatic volume. While TRUS is still used to estimate prostate volume, more recently prostate Magnetic Resonance Imaging (MRI) has also come into favor. Current literature indicates MRI is more accurate than TRUS for prostate size, but few studies exist, and whether differences in the two are clinically significant remains uncertain. The purpose of this study was to compare prostate volumes from TRUS and MRI to gross specimens after prostatectomy. Methods: Patients who underwent radical prostatectomy for prostate cancer between 2017–2022 were identified. TRUS and MRI measurements closest to the date of surgery were obtained. These were compared to gross prostate specimens after surgery, which was considered the gold standard. All TRUS volumes were recorded by a urologist during prostate biopsy using the ellipsoid formula (L*W*H*(π/6)). MRI measurements were done by a radiologist also using the ellipsoid formula. The weight and dimensions of gross specimens were measured by a pathologist. Ellipsoid formula was then used to calculate final volume measurements in gross specimens. Paired samples t-test was used to compare averages of TRUS and MRI to the gross specimen. Results: 83 patients were included in the study with an average age at prostatectomy of 65.47 years. TRUS volume significantly differed from gross specimen volume by an average of –4.56 mL (p=0.017) and gross specimen weight by -14.31 g (p <0.001). MRI volume was not significantly different from gross specimen volume, by an average of –0.56 mL (p=0.771) and was significantly different from prostate weight by –10.32 g (p <0.001). When compared to one another, TRUS and MRI significantly differed on average by –4 mL (p=0.033). Conclusions: MRI is more accurate than TRUS to estimate prostate volume. Both MRI and TRUS underestimate prostate weight. Urologists should be aware of potential inaccuracies when assessing preoperative prostate volume with TRUS, and recognize MRI is the best predictor of size. While imperfect, TRUS is still relatively accurate measuring prostate volume, and given its ease of availability and cost, we feel it is a useful modality to determine prostate size.