Two studies were conducted to evaluate the effects of sodium diformate in swine diets. For experiment 1, 360 barrows (DNA 200 × 400; initially 5.9 ± 0.06 kg) were used in a 38-d study. At weaning, pigs were randomly assigned to pens with 5 pigs per pen. Each pen was allocated to 1 of 6 treatments with 12 pens per treatment. Treatments were formulated to provide none, 0.40, 0.60, 0.80, 1.00, or 1.20% sodium diformate added at the expense of corn. Diets were fed in 3 phases: phase 1 from weaning to d 9, phase 2 from d 9 to 24, and phase 3 from d 24 to 38. From d 0 to 24 (phases 1 and 2), increasing sodium diformate increased (linear, P = 0.001) gain-to-feed (G:F). However, sodium diformate did not affect average daily gain (ADG) or average daily feed intake (ADFI). From d 24 to 38 (phase 3) and overall (d 0 to 38), there was no evidence of differences due to increasing sodium diformate for any growth response criteria. There was no evidence for differences in fecal dry matter (DM) on d 9. However, fecal DM decreased (linear, P < 0.05; quadratic, P = 0.097) as sodium diformate increased on d 24. In experiment 2, 2,200 pigs [Duroc sire (PIC 800 or DNA 600) × PIC Camborough; initially 24.2 ± 0.30 kg] were used in a 117-d growth trial. Pens of pigs (25 pigs per pen) were randomly assigned to 1 of 4 treatments with 22 pens per treatment. Treatments were formulated with additions of none, 0.25, 0.50, or 0.75% sodium diformate. Diets were fed in 6 phases from 24 to 141 kg. For period 1 (d 0 to 32), ADFI tended to decrease then increase (quadratic, P = 0.081) with increasing sodium diformate whereas G:F increased then decreased (quadratic, P < 0.001) with increasing sodium diformate. For period 2 (d 32 to 60), there was no evidence for differences in ADG or ADFI; however, there was a tendency for G:F to increase then decrease (quadratic, P = 0.093) with increasing sodium diformate. From d 60 to 93, increasing sodium diformate increased (linear, P < 0.01) ADG and ADFI. From d 93 to 117, increasing sodium diformate increased (linear, P < 0.05) ADG, ADFI, and G:F. Overall (d 0 to 117), pigs fed increasing sodium diformate had increased (linear, P < 0.01) ADG and a tendency for increased (linear, P = 0.075) ADFI; however, there was no evidence for differences in G:F. There were no treatment differences for any carcass characteristic. In summary, increasing sodium diformate may increase G:F in the early nursery and improve ADG after d 60 (~82 kg) in the finishing period.