Early design stage decision making proceeds very quickly, and strategic decisions regarding the feasibility of natural ventilation must be supported by speedy and reliable evaluation. There are a limited number of such early stage design tools for evaluating natural ventilation options, and none of them are widely used in practice. This paper will present a simple steady-state tool which was initially developed as an in-house tool within a UK based architecture and environmental design practice. The tool has been applied in the development of a wide range of projects, and has proved to be reliable and quick to use in evaluating the broad feasibility of natural ventilation options. It is also an innovative educational tool which has been used widely by post-graduate students in exploring the implications and opportunities for natural ventilation in different locations and for different building types. This tool has recently been further developed for more widespread use and application. The objective was to expand the range of generic airflow strategies that can be explicitly evaluated, extend the geographic applicability of the tool, incorporate a more user-friendly graphic interface, and release it in a web-based platform to be accessible from any device. A comparative evaluation of the attributes of different existing tools, a survey of user experience and a critical evaluation of potential improvements to the detailed structure of the tool, has formed the basis of the further development of this tool. It is envisaged that these developments will make it a more widely used professional tool and facilitate the design and evaluation of proposals for low carbon naturally ventilated buildings worldwide.