From a large number of experimental data, a formulation was developed for predicting the smoke number (SN) as measured in gas-turbine exhausts according to the well-established Society of Automotive Engineers standard. Three different scales of the same combustor were used with inlet temperature and pressure ranging from 300 to 600 K and from 0.1 to 0.9 MPa, respectively. The formulation is based on the residence time that is calculated from the mass e ow rate, density, and the volumes of the primary and secondary zones of the combustor. The reaction rate has an Arrhenius form with the equivalence ratio to take into consideration the air and fuel e ow rates. All of the required parameters can be evaluated from desired operating conditions. Nineteen different types of fuel were used, varying from a parafe nic mixture to a pure aromatic compound. The fuel is characterized by its calorie c value and the hydrogen mass fraction. With this wide range of fuels burned in the experiments, giving a SN variation from 0 to 100, the accuracy of the prediction (standard deviation of 40% on the relative error to experimental values for each scale and 60% when all scales are combined ) is acceptable for most purposes. Measured SN values already have a 20% error because of the commonly accepted variability of the technique. The formulation should be particularly useful in assessing the efe ciency of new systems for smoke reduction or in calculating the SN from older experimental data where it was not measured.