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Non-functional requirements
A non-functional requirement defines how well a system must perform rather than what it does, covering qualities like speed, security, reliability, scalability, and usability. Non-functional requirements set the standards a product has to meet, such as loading in under two seconds, and they often decide whether users actually accept it.
What it means
If functional requirements are the features, non-functional requirements are the conditions those features have to live up to. They rarely show up in a demo, but they are what people feel: the page that loads instantly, the app that does not fall over under load, the system that keeps data safe. Teams sometimes call them quality attributes, or 'the ilities', for reliability, scalability, maintainability, and the rest.
They are easy to overlook and expensive to bolt on late. A system built without a clear performance or security target often has to be reworked once real usage exposes the gap. Writing non-functional requirements as measurable numbers, not adjectives, is what makes them testable. 'Fast' is an opinion. 'Responds within 500 milliseconds for 95 percent of requests' is a requirement.
In practice
A hospital scheduling system has a functional requirement to book appointments. Its non-functional requirements say the system must stay available 99.9 percent of the time and encrypt patient data at rest. Miss those, and the booking feature technically works while still being unusable in a real hospital.
Häufig gestellte Fragen
What are examples of non-functional requirements?
Why are non-functional requirements often missed?
Manage requirements the modern way
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