There are five sources of validity evidence that are content, response process, internal structure, relation to other variables, and consequences. Content validity is the extent of a measurement tool represents the measured construct and it is considered as an essential evidence to support the validity of a measurement tool such as a questionnaire for research. Since content validity is vital to ensure the overall validity, therefore content validation should be performed systematically based on the evidence and best practice. This paper describes a systematic approach to quantify content validity in the form of content validity index based on the evidence and best practice.