This study was conducted to test the hypothesis that the values of standardized total-tract digestibility (STTD) of phosphorus (P) measured in individual feed ingredients are additive when used in diet formulation for swine. Eight crossbred barrows with an initial BW of 30.5 ± 1.5 kg were individually housed in metabolism cages equipped with a feeder. The pigs were randomly allotted to a replicated 4 × 4 Latin square design with 4 dietary treatments and 4 periods. Three diets were formulated by mixing cornstarch, sucrose, and soybean oil with SBM (50%), wheat (70%), or both (30.6% of SBM and 55.7% of wheat), as the source of P in each diet. A P-free diet was also formulated to estimate the basal endogenous loss of P. All experimental diets were formulated to maintain constant total calcium to total P ratio of 1.2:1 except the P-free diet. Each experimental period consisted of a 4-d adaptation period and a 5-d collection period. Feces were collected using the marker-to-marker procedure. In the mixed diet, a relative contribution of P from SBM and wheat was 51.9 and 48.1%, respectively. The ATTD of P for SBM and wheat was measured to be 40.9 and 41.8%, respectively, and the measured ATTD of P in the mixed diet was greater (45.1 vs. 41.3%; P < 0.05) than the predicted ATTD of P. The determined STTD of P for SBM and wheat was 46.4 and 49.3%, respectively, and the measured STTD of P in the mixed diet was not different (49.7 vs. 47.8%; P = 0.26) from the predicted STTD of P. In conclusion, the ATTD values of P are not additive whereas STTD values of P may be additive when formulating swine diets.