The magnetopause is the boundary where the reduced solar wind dynamic pressure is equal to the magnetic pressure of the Earth's outer magnetosphere. With hundreds of magnetopause crossings identified from the THEMIS data, we estimate a ratio (f) of the compressed magnetic field just inside the subsolar magnetopause to the purely dipolar magnetic field. Previous theoretical studies reported that the ratio f was nearly independent of the subsolar standoff distance (r(0)). Here we report that the ratio f is linearly proportional to r(0) for both northward and southward interplanetary magnetic field (IMF). The proportionality constant for southward IMF is larger than that for northward IMF, implying that the compression level of the magnetic field by inward magnetopause for southward IMF is smaller than that for northward IMF.