In single point incremental forming (SPIF), the final thickness of a deformed sheet can be predicted by the sine law. Therefore, the formability in SPIF can be expressed as the maximum wall angle (θ_max) that a sheet would endure without fracturing. In the present study, two tests were carried out in order to evaluate the formability of an aluminum sheet. In the first test, conical frustums and square pyramids, a set of each, were produced by systematically varying the wall angle in order to investigate θ_max. In the second test, four conical frustums, each having varying wall angle, designed by revolving different curved lines were formed to fracture. The results revealed that the value of θ_max obtained from the former test was smaller than those obtained from the latter one. Moreover, a variation among the values of θ_max obtained from the parts of the second test was also found. Since the first test shows the minimum possible value of θ_max, it should be employed in combination with the second test so as to minimize the number of experiments required.