The United Nations’ Sustainable Development Goals (SDGs) are an important global framework which provides a shared vision for a more sustainable future for all people and the planet. In the last five years, citizen science as a discipline has paid increasing attention to the SDGs and the contributions that citizen science could make towards their achievement. This paper presents a collection of 51 questions and a corresponding set of answers which can be used by citizen science projects to self-assess their impact towards the SDGs. The questions and answers were originally derived from the official UN description of the SDG targets and indicators and were refined through a workshop and series of nine interviews with citizen science project coordinators. The outcomes of the workshop and interviews reveal the challenges of assessing impacts towards the SDGs in a way which is relevant to the majority of citizen science projects. In many cases, the wording of the SDGs had to be altered to make sense in the context of citizen science. The final set of questions and answers are structured to reflect two pathways of impact: citizen science contributing to the (official) monitoring of the SDGs, and citizen science contributing to the direct achievement of the SDGs.