Anthropogenic underwater sounds represent a potential risk to aquatic organisms. Many regulators require this risk to be assessed before allowing a sound-producing activity to proceed. Regulators typically set exposure criteria for a range of acoustic parameters and require the assessment to address whether a given acoustic metric would exceed a specified allowable threshold. While the value of the threshold is usually clear, the procedure required to calculate the metric sometimes lacks sufficient detail, leading to uncertainties and potential inconsistencies in interpretation. Procedures are described that enable intra- and inter-project consistency in use, processing, and reporting of metrics. Quantities derived from sound pressure and sound particle motion are considered, providing metrics relevant to assessments for fish, aquatic invertebrates, and aquatic mammals. Specific metrics for which procedures are described include transient duration, zero-to-peak quantities, mean-square and time-integrated-squared quantities (e.g., sound exposure), and their spectral densities. The metrics are relevant to ambient sound and to specified activities such as drilling, pile-driving, seismic imaging, or dynamic positioning. Issues addressed include the specification of fractional octave and fractional decade bands, and the harmonization of units and reference values used for reporting. Continuing efforts on standards for the characterization of underwater sound sources are described.Anthropogenic underwater sounds represent a potential risk to aquatic organisms. Many regulators require this risk to be assessed before allowing a sound-producing activity to proceed. Regulators typically set exposure criteria for a range of acoustic parameters and require the assessment to address whether a given acoustic metric would exceed a specified allowable threshold. While the value of the threshold is usually clear, the procedure required to calculate the metric sometimes lacks sufficient detail, leading to uncertainties and potential inconsistencies in interpretation. Procedures are described that enable intra- and inter-project consistency in use, processing, and reporting of metrics. Quantities derived from sound pressure and sound particle motion are considered, providing metrics relevant to assessments for fish, aquatic invertebrates, and aquatic mammals. Specific metrics for which procedures are described include transient duration, zero-to-peak quantities, mean-square and time-integrated-...