A model has been developed for calculating the electrical resistivity of commercial aluminium alloys from composition and heat treatments using the Matthiessen rule. The model is based on the approximation that the solubility of the alloying elements in heat treated alloys is equivalent to the equilibrium solubility at a higher temperature. These temperatures were determined from heat treatment data. The resistivity of a wide range of commercial alloys was calculated using the model, showing an agreement with most observed resistivity values of within 3 n Omega m, except for alloys with special composition characteristics. According to the model, magnesium and manganese are important contributors to the resistivity for all main groups of alloys. In heat treated alloys the contribution of precipitates is 6-17% of the total resistivity. Thermal conductivity was calculated for alloys given in the literature using the Wiedeman-Franz law and the calculated resistivity. The calculated thermal conductivity agreed with the experimental values for the AlMg-alloys, but it was lower than the experimental values for pure aluminium and the AlCu-alloys in the annealed condition.