Although the principal dosimetric quantity in computed tomography (CT) can be assessed using a pencil ionization chamber with an active length of 100 mm, standard CT dosimetry phantoms of polymethylmethacrylate (PMMA) , and plates of aluminum, most facilities do not possess the requisites. We present a practical method of estimating CTDI(100, c), CTDI(100, p) and the half-value layer (HVL) from CTDI(100, air), which is measured parallel with the axis of rotation of the scanner to free-in-air. The three data chosen for this method of estimation were as follows: 1) the relation of HVL to CTDI(100, air) per radiographic exposure (mAs); 2) the relation of HVL to CTDI(100, c) per CTDI(100, air); 3) the relation of HVL to CTDI(100, p) per CTDI(100, air). The data were based on the measured values of six CT scanners, so as to avoid dependence on the technical characteristics of a specific manufacturer. The estimated value has a possible maximum uncertainty of 20%, although this method of estimation is practical for dose assessment.