The premise is suggested that attenuation in CT scanning should be expressed in terms of physical density rather than CT numbers. A simple experiment was performed to establish a calibration curve of physical density vs. CT number which, within the constraints of beam hardening and other sources of CT number variation and drift, is unique to the specific scanner being calibrated. By expressing attenuation values in terms of physical density, the mass in grams of any organ or tumor can be calculated in vivo, from its volume determined also by CT. As an example, density of bone becomes additional information with any CT scan done for another purpose that includes plane(s) across a skeletal structure.