A proposed method to obtain the dose for Computed Tomography was evaluated on long a phantom, made of polyethylene. This phantom is 600 mm large. Adopting an empirical characterized function , a tendency of the asymptotical grow of the dose by the increase of the scanning length can estimate the dose in an infinite scanning length for a single axial rotation of the x-ray tube. The curve of was estimated for different clinical protocols: tension, pitch, head/body, and center/peripheral measurements. The ultrahigh molecular weight polyethylene phantom of 600 mm length, named ICRU/AAPM phantom, was proposed by a partnership between ICRU and AAPM (Task Group 200). This length provides a higher scanner length then PMMA phantom for , whose measurement is 100 mm. The measures of ICRU/AAPM phantom were performed on General Electric, Toshiba, and Philips scanners. The measurement was performed on a thimble ion chamber inserted in the center or periphery of the phantom. Using the measurements, functions (equilibrium dose, equilibrium length and scatter fraction) were obtained. A single scan has been demonstrated to be sufficient to obtain the function.