This study investigates the dosimetry methodology proposed by the American Association of Physicists in Medicine (AAPM) task group 111 and compares with the computed tomography dose index (CTDI) method and the SEDENTEXCT DI method on one clinical multislice CT and two dental cone beam CT (CBCT) scanners using adult, adolescent, and child head phantoms. Following the AAPM method, the normalized (100 mAs) equilibrium doses ([Formula: see text]) for Toshiba Aquilion One MSCT computed using dose measurements from the central hole of the phantom ([Formula: see text]), the peripheral hole of the phantom, ([Formula: see text]), and by the [Formula: see text] equation ([Formula: see text]) are in the range from 20 to 25 mGy. For i-CAT Next Generation dental CBCT, the normalized [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] by the two SEDENTEXCT DI methods are in the range from 12 to 15 mGy. Fitting the AAPM equation is not possible for the limited scan lengths available on the CS 9300 dental CBCT. This study offers a simple CTDI-like measurement that can approximate the AAPM [Formula: see text] in clinical CBCT scanners capable of providing four or more scan lengths.