In systems engineering and requirements engineering, a non-functional requirement is a requirement that specifies criteria that can be used to judge the operation of a system, rather than specific behaviors. This should be contrasted with functional requirements that define specific behavior or functions. The plan for implementing functional requirements is detailed in the system design. The plan for implementing non-functional requirements is detailed in the system architecture.
Broadly, functional requirements define what a system is supposed to do whereas non-functional requirements define how a system is supposed to be. Functional requirements are usually in the form of "system shall do <requirement>", while non-functional requirements are "system shall be <requirement>".
Non-functional requirements are often called qualities of a system. Other terms for non-functional requirements are "constraints", "quality attributes", "quality goals", "quality of service requirements" and "non-behavioral requirements". Informally these are sometimes called the "ilities", from attributes like stability and portability. Qualities, that is non-functional requirements, can be divided into two main categories:
- Execution qualities, such as security and usability, which are observable at run time.
- Evolution qualities, such as testability, maintainability, extensibility and scalability, which are embodied in the static structure of the software system.
A system may be required to present the user with a display of the number of records in a database. This is a functional requirement. How up-to-date this number needs to be is a non-functional requirement. If the number needs to be updated in real time, the system architects must ensure that the system is capable of updating the displayed record count within an acceptably short interval of the number of records changing.
Sufficient network bandwidth may be a non-functional requirement of a system. Other examples include:
- Audit and control
- Availability (see service level agreement)
- Capacity, current and forecast
- Configuration management
- Dependency on other parties
- Disaster recovery
- Efficiency (resource consumption for given load)
- Effectiveness (resulting performance in relation to effort)
- Emotional factors (like fun or absorbing)
- Environmental protection
- Extensibility (adding features, and carry-forward of customizations at next major version upgrade)
- Failure management
- Fault tolerance (e.g. Operational System Monitoring, Measuring, and Management)
- Legal and licensing issues or patent-infringement-avoidability
- Network topology
- Open source
- Performance / response time (performance engineering)
- Platform compatibility
- Quality (e.g. faults discovered, faults delivered, fault removal efficacy)
- Recovery / recoverability (e.g. mean time to recovery - MTTR)
- Reliability (e.g. mean time between failures - MTBF)
- Resource constraints (processor speed, memory, disk space, network bandwidth, etc.)
- Response time
- Safety or Factor of safety
- Scalability (horizontal, vertical)
- Software, tools, standards etc. Compatibility
- Usability by target user community
See also 
- ISO/IEC 9126
- Requirements analysis
- Functional requirements
- Usability requirements
- Non-Functional Requirements framework
- Stellman, Andrew; Greene, Jennifer (2005). Applied Software Project Management. O'Reilly Media. p. 113. ISBN 978-0-596-00948-9.
- Wiegers, Karl E. (2003). Software Requirements, Second Edition. Microsoft Press. ISBN 0-7356-1879-8.
- Young, Ralph R. (2001). Effective Requirements Practices. Addison-Wesley. ISBN 978-0-201-70912-4.
Templates and examples
Modeling non-functional properties in SOA
Agile non-functional requirements