The Hounsfield units into density conversion for a commercial treatment planning system (TPS) is discussed. The impact of an average calibration table provided by the manufacturer on computed doses was investigated when a customer calibration could not be included in the TPS. The maximum error determined on the computed monitor units per Gy was found to be about 2%, being below 1% on average. The relative importance of the parameters used for CT image acquisition is also discussed. The applied voltage was the most relevant parameter leading to errors in the reconstructed Hounsfield numbers of about 300 units for high densities.