As the development of nature-based solutions (NbS) increases globally, it is important to ensure that projects meet the objective of delivering benefits for biodiversity, alongside tackling societal challenges. However, this is challenging because most NbS projects do not directly monitor ecological outcomes, and those that do often focus on a limited set of metrics. Our study provides practitioners with a framework for selecting optimum metrics for assessing above- and below-ground ecological outcomes of NbS relevant to the location in which they are being implemented. We summarise the relevance of each metric to biodiversity or soil health and provide standardised methodologies for collecting data to support ecological monitoring protocols for NbS projects. The information on each metric is freely available as a searchable online database designed for UK practitioners, but with wider applicability.