This article reports the calibration procedure for a commercial Solartron 7835 densimeter which is currently used as “reference” instrument in an automatic apparatus for the calibration of on-line densimeters. Three different materials (fluids), whose densities are known at the usual working condition of pressure and temperature, are used for characterizing the reference densimeter. The procedure applied at the INRIM to produce accurate parameter estimates in the densimeter calibrations is described in this papers. It is based on the application of the weighted least square method (WLS), considering together with the uncertainties of the dependent and independent variables also those of the model. The model analysis performed by a Chi-square test and the expected uncertainty computed over the calibration range have shown to be fully consistent with the experimental data.