The most significant energy consumers in energy related industries are boilers and other gas-fired systems. Combustion efficiency term commonly used for boilers and other fired systems and the information on either carbon dioxide (CO2) or oxygen (O-2) in the exhaust gas can be used. The aim of this study is to develop a simple-to-use predictive tool which is easier than the existing approaches less complicated with fewer computations and suitable for combustion engineers for predicting the natural gas combustion efficiency as a function of excess air fraction and stack temperature rise (the difference between the flue gas temperature and the combustion air inlet temperature). The results of the proposed predictive tool can be used in follow-up calculations to determine relative operating efficiency and to establish energy conservation benefits for an excess air control program. Results show that the proposed predictive tool has a very good agreement with the reported data where the average absolute deviation percent is 0.1%. It should be noted that these calculations are based on assuming complete natural gas combustion at atmospheric pressure and the level of unburned combustibles is considered negligible. The proposed method is superior owing to its accuracy and clear numerical background, wherein the relevant coefficients can be retuned quickly for various cases. This proposed simple-to-use approach can be of immense practical value for the engineers and scientists to have a quick check on natural gas combustion efficiencies for wide range of operating conditions without the necessity of any pilot plant set up and experimental trials. In particular, process and combustion engineers would find the proposed approach to be user friendly involving transparent calculations with no complex expressions for their applications to the design and operation of natural gas-fired systems such as furnaces and boilers. (C) 2010 Elsevier B.V. All rights reserved.