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: l) 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.