Although fractal method performs well in calculating the swelling deformation of bentonite, it does not provide an effective method to calculate coefficient K, which limits the application of the fractal theory. Because both the DDL theory and the fractal theory are applicable in analyzing the behavior of compacted bentonite, a method is proposed to determine coefficient K in the fractal theory based on the DDL theory, by which a value of 9.15 is determined for coefficient K of commercial bentonite. N2 adsorption tests are the performed on the commercial bentonite, and the measured isotherm data are used to determine a surface fractal number of 2.65. Then, the so-determined K and the surface fractal number are used to calculate the maximum swelling deformation through the fractal method, and the calculations are compared to the experimental measurements. It is shown that the results of the fractal method agree well with the experimental data, especially when the applied loading is large and the swelling deformation is small. It is also shown that the fractal theory yields better results than the DDL theory.