The success of catheter ablation of atrial fibrillation (AF) is highly dependent on a preprocedural assessment of the size and shape of the left atrium. The most precise method to determine left atrial (LA) volume using computed tomography requires manually tracing the LA area of each cross-sectional image. This is a labor-intensive and time-consuming technique. The purpose of this study was to compare LA volume derived using the “gold-standard” multiple-slice technique with LA volume estimated using 3 orthogonal LA dimensions in patients with AF. The patient population was composed of 100 patients referred for catheter ablation of AF (87 men, mean age 57 ± 12 years). AF was paroxysmal in 49 patients and persistent in 51. Each patient underwent computed tomography before catheter ablation, and LA volume was measured using the 2 methods. The mean LA volume measured using the multiple-slice technique was 136 ± 46 ml. According to the simpler estimation approach, the mean LA volume was 112 ± 41 ml. A close correlation was noted between atrial volumes determined using the 2 methods (r = 0.91, p <0.001). There was a mean underestimation of LA volume by the estimation technique of 17 ± 13%. In conclusion, the results of this study reveal that LA volume determined using an estimation approach correlates closely with true LA volume as determined using the gold-standard multiple-slice approach. This estimation approach underestimates true LA volume by approximately 20%.