CUBIC, the cosmic unresolved x ray background instrument using CCDs, is instrumented to make moderate resolution x-ray spectral measurements of diffuse targets at spatial scales of a few degrees. While the energy range is nominally 200 eV - 10 keV, the CCDs have been designed to maximize the soft x ray performance by using novel structures. A two part aperture increases the area-solid angle product above 1 keV to maximize sensitivity to the cosmic component of the diffuse x ray background. Here we report preliminary results of our preflight laboratory calibrations performed at Penn State of the dark current, readnoise, nonlinearity, charge transfer efficiency, energy resolution, and quantum efficiency of the two flight CCDs. We also discuss calibration of the detector field of view and the preliminary area-solid angle product of the instrument.