In psychology and sociology, a trust metric is a measurement of the degree to which one social actor (an individual or a group) trusts another social actor. Trust metrics may be abstracted in a manner that can be implemented on computers, making them of interest for the study and engineering of virtual communities, such as Friendster and LiveJournal.

Trust escapes a simple measurement for several reasons, be it its unclear definition, its complexity, subjectivity or the fact that it is a mental process, unavailable to instruments. There is a strong argumentation against the use of a simplistic metrics to measure trust that validly points to the complexity of the process and the embeddedness of trust that makes it impossible to capture trust in a clear form. For a detailed discussion about different trust metrics see.

There is no generally agreed set of properties that make a particular metric better than others, as each metric is designed to serve different purposes, e.g. provides certain classification scheme for trust metrics. Two groups of trust metrics can be identified:

Empirical metrics focusing on supporting the capture of values of trust in a reliable and standardised way
Formal metrics that focus on formalisation leading to the ease of manipulation, processing and reasoning about trust; formal metrics can be further classified depending on their properties.