This study proposes a method to accurately estimate the phantom scatter factor (S_p) of arbitrary rectangular fields. We measured output doses in water and air; these measured values were based on square fields and a limited number of symmetric rectangular fields using 4 MV and 10 MV X-rays of a Varian Clinac-iX. We calculated S_p from these measured values. Then, using these S_p values, we estimated equations of S_p on square fields consisting of the primary dose, Day’s scatter, and forward scatter. This equation may be used to estimate the S_p value on a square field, but it cannot estimate the S_p value on a rectangular field. We investigated the calculation method for an equivalent square of a rectangular field. As a result, this study’s calculation method for an equivalent square, the area ratio correction method, was more accurate than the conventional Bjärngard’s method. Therefore, when using the approximate equation of S_p on a square field and the equivalent square calculated by the area ratio correction method, a S_p value of an arbitrary rectangular field may be accurately estimated.