An analytic model for the performance of cone beam microtomography is described. The maximum power of a microfocus x-ray source is assumed to be approximately proportional to the focal spot size. Radiation flux penetrating the specimen is predicted by a semi-empirical relation which is valid for x-ray energies less than 20 keV. Good signal to noise ratio is predicted for bone specimens of .1 to 10 mm when scanned at the optimal energy. A flux of about 1 x 10^10 photons/mm^2/s is identified for .2 mm specimens. Cone beam volumetric microtomography is found to compare favorably with synchrotron based methods.