Left atrial (LA) size assessment by anteroposterior dimension is limited in accuracy. Conventional 3-dimensional (3D) reconstruction has been validated, but the process is time-consuming and the 3D system is not widely available. We developed an algorithm to simplify 3D reconstruction of LA on the basis of 3 standard apical views, tested it in 44 hemodynamic stages of 8 open-chest dogs, and compared it with LA volumes assessed by conventional 3D reconstruction. Simplified 3D reconstruction provided an accurate LA volume measurement (y = 0.93x + 0.7, r = 0.95, standard error of the estimate [SEE] = 3.6) with more than 60% of time saved. Cubic equation of anteroposterior dimension and biplane modified Simpson's method were less accurate (for biplane modified Simpson's method, y = 0.8x + 2.6, r = 0.88, SEE = 5.0; for cubic equation of anteroposterior dimension, y = 0.65x + 2.6, r = 0.76, SEE = 8.2). Without the need for a 3D-imaging acquisition tool, simplified 3D reconstruction can be applied in the clinical setting for LA size quantitation with significant time saved.