Born circular startups play a pivotal role in implementing high-level circularity to fight resource depletion and address climate change. In developing circular business model innovation, circular startups face the challenge of lacking legitimacy, which can be approached by certifications provided by concepts like Cradle-to-Cradle. As there is currently no supporting tool to develop such business models, this paper employs a design science research approach and proposes a Cradle-to-Cradle (C2C) Business Model Tool. The tool is developed based on a literature review, feedback from 42 users in six demonstration workshops and is finally tested with eight startups. It succeeds in aiding born circular startups in navigating the venturing process with a focus on high-level circularity. In this paper, C2C principles developed for the product level are theoretically linked to the business model level. The tool addresses elements such as partnerships, communication, and action-taking, thereby contributing to the literature on circular business model innovation and serving as a practical resource for born circular startups.