The goal of the work was to develop a simple yet reliable and computationally inexpensive mathematical tool for the performance prediction of 'rocket' type natural draft direct combustion stoves with unshielded pot. The work included development of a novel heat and mass transfer model for natural draft biomass cookstoves and its integration with Excel (R) spreadsheet to develop a user-friendly mathematical tool. The results from the mathematical tool were validated against the experimental data set from literature. A new operational parameter named 'Inlet area ratio' was identified, and its effects on the stove performance were investigated. It was concluded that this newly identified parameter has a considerable effect on the performance of a natural draft stove, and the findings are likely to change the future cookstove modeling and design approach. (C) 2015 Elsevier Ltd. All rights reserved.