Requirements engineering deal with elicitation, analysis, communication and validation of the requirements. As early as errors are identified it is easy to fix them compared to later identification of errors. It is obvious that the cost of fixing these errors in initial stages is lower

than fixing them in later stages of software development. In order to eliminate errors there should be some measurement. Metrics provide a quantitative measurement of certain attributes.
We know the main quality parameters of requirements are: Understandability, Completeness, Testability and Volatility. Metrics of these parameters can give us a quantitative measurement of requirement's quality:
1. Understandability: % of requirements is readable or understandable by the reviewers or next level users. Metrics is (Number of understood requirements) / (Total number of requirements). E.g. if total number of requirement is 40 and 30 requirements are understandable by the stakeholders then understandability of those requirement is 75%.
2. Completeness: % of requirements is complete (no item left to the requirement specification). Metrics is (Number of complete requirements) / (Total number of requirements). E.g. if total number of requirement is 40 and 35 requirements are complete then completeness of those requirement is 87.5%.
3. Testability: % of requirement is testable. Metrics is (Number of testable requirements) / (Total number of requirements). E.g. if total number of requirement is 40 and 38 requirements are testable then testability of those requirement is 95%.
4. Volatility: % of requirement changes. Metrics is (Number of changed requirement) / (Total number of requirements). E.g. if total number of requirement is 42 and 5 requirements are changed then volatility of those requirement is 11.9%.
Note: Change requirement = Modified requirements + New requirements.
Other requirement metrics can be developed as per the company need. GQM (Goal, Question, Metrics) is a popular method for metrics development.