The industrial transition towards a more sustainable Circular Economy (CE) emphasizes the need for products that align with CE principles and robust methods for assessing how circular they are. However, ambiguities between metrics vs. indicators are not yet addressed in the current discussion of product circularity assessment (PCA). Most PCA methods also lack clarity in the metrics used, including their selection and description, which leads to confusion and limited industry adoption. To address these gaps, this paper proposes clear definitions for indicators and metrics and adapts standards to establish a structured approach for developing PCA metrics. This process involves establishing the objective or purpose of each metric, identifying candidate metrics from literature sources or existing PCA methods, and defining them with detailed context (e.g., description, formula, range) and content information (e.g., notes, references, standards/guidelines). To effectively compile and communicate the developed metrics, an Excel-based database consisting of attributes, indicators, and metrics (AIM) was developed and presented. Over 80 relevant metrics for measuring product circularity were identified, characterized, and organized under the AIM database. The standards-guided approach and database proposed in this paper formalize the development of metrics for PCA and aid data collection processes, enhancing their practical application in the industry. The findings in this paper further contribute to the successful transition toward product-level CE.