The experimental form factor (EFF) is the ratio of tree volume to the volume of a cylinder that has the same diameter at breast height and is 3 m (K) higher than the tree, which is an important method to estimate stand volume. To estimate the stand volume more accurately and simply, the proposed approach considered form factors. Based on 14 clear-cut sample plots of spruce-fir mixed stands at Changbai Mountains, Jilin, China, an EFF stand volume formula was established using mathematical deduction and was weighed against the estimating precision by a 2-entry table method. The results demonstrated that the model had a smaller deviation, and better prediction precision and goodness of fit by applying the EFF formula than the 2-entry table formula. Through an independent-samples t-test and F test, no significant differences were observed between the real stand volume and the stand volume estimated by the EFF formula. The EFF volume formula was first deduced from the theoretical stem curve formula with the breast height form quotient (BFQ). In a comparison of results obtained by the classical approach, EFF values for 11 species applying the theoretical derivation marginally differed. The EFF was verified to be affected only by species. Item K was certified to be constant term with a coefficient of variance of 0.02.