Our goal was to understand why it is difficult to achieve reliable valve competence after aortic valve-sparing surgery, and to propose quantitative data aimed at improving the outcome of the procedure. Valve-sparing procedures were performed in patients with dilated aortic roots and aortic regurgitation, and reproduced in physical models to explore what should be the restored dimensions of the aortic root and leaflets for valve sparing to be successful. In parallel, a three-dimensional geometric model of the aortic valve was tested to evaluate its capability to predict the annulus diameter, sinotubular junction diameter, valve height, and leaflet free-edge length and height in competent spared valves. Valve sparing resulted in more or less severe residual regurgitation in all the patients considered. Successful valve-sparing was achieved in vitro by making further changes to the annulus diameter, the leaflet free-edge length and/or graft size. The changes needed were effectively predicted by the geometric model. Tabulated valve dimensions allowing restoration of competence were generated for convenient use by surgeons. A quantitative approach to aortic valve sparing is proposed, putting emphasis on the functional characteristics of the restored valve geometry.