This paper seeks to contribute to sustainable business model innovation (SBMI) literature. It aims to do so by putting forward a relatively simple tool that simultaneously calculates the financial value alongside sustainability impact based on the Sustainable Development Goals (SDGs) of a proposed business model innovation. For small businesses to validate the outcome of a proposed SBMI, some form of sustainability measurement will be necessary. Simple tooling specifically aimed at small businesses do not exist. We address this gap in how to predict or create a prognosis of the combined financial and sustainability effect of a proposed business model (BM) in a frugal (easy, time and knowledge effective) and effectual (allowing for iterations, available means and calculating affordable loss) manner. The tool is called the Pos-FSBC (Positive Financial and Sustainability Business Case). The instrument is a calculation model in Excel where users insert a limited number of numerical variables. Alongside financial variables the tool uniquely links the key variable ∆ SDG to the expected quantity sold, it then calculates the contribution to the SDGs in a relevant and measurable unit. By being successful with a sustainable innovation, the tool helps businesses drive out nonsustainable competitors. The tool has been iteratively developed and tested in several students’ projects and in a pilot with practitioners. Based on the findings we propose more iterations to develop an understanding whether the tool inspires business change and if so how.