Background Patients needing silicone gel–filled breast implant surgical revision often do not have information as to the size of their implants. Clinical estimates may be erroneous and an accurate, in vivo method of determining breast implant volume would be very useful in planning surgery. Objective Our objective was to determine whether magnetic resonance imaging (MRI) scans of the breast in combination with a computer-assisted detection (CAD) system could provide accurate estimation of implant volumes in patients with silicone gel–filled implants, using known implant volumes for comparison. Methods All MRI scans were performed for medical necessity before implant revision surgery or for follow-up. Scans and silicone implant volume calculations were performed in 20 patients (39 breasts). Fifteen patients (n = 27 breasts) had known implant volumes. The MRI images were analyzed blindly by a single radiologist using the image processing features of an MRI CAD system (DynaCAD; Invivo, Birmingham, MI) in a novel way. Computed implant volumes were compared to known implant volumes obtained from documented preoperative notes (n = 17) or following implant removal via implant markings (n = 10). Results Average deviation of the 27 known implant volumes from those calculated by MRI DynaCAD analysis was only 0.82% (SD, 3.95%). In four of 27 breasts (14.8%), MRI calculations matched the clinical volumes exactly. The widest volume deviation range for all implants was +11.4% to −4.44%. A variety of implant types were found, with 11 confirmed ruptures. Conclusions The calculation of silicone gel–filled breast implant volume using breast MRI scans and a commercially-available CAD system appears to be sufficiently accurate that it may have significant clinical benefit in planning revision implant surgery. Calculations can be easily obtained in five minutes. To our knowledge, this is the first report describing this method to reliably measure silicone implant volumes preoperatively.