Functional requirements say what the system does; non functional ones say how well it has to do it. Performance, availability, security, accessibility, and data retention are typical. They matter because they are architectural: you cannot bolt on a response time target or a seven year audit trail late without rework. Get numbers on them early, because acceptable is not a testable requirement.
Why interviewers ask this
This is a fundamentals question with a practical edge. Interviewers want the definitions right and then evidence that you push for measurable thresholds instead of adjectives. The insight they reward is that non functional requirements drive architecture and are expensive to retrofit, plus the awareness that they are the requirements most often deferred until testing, which is exactly when they turn into an emergency.
How to structure your answer
- Define both in one sentence each.
- Explain why non functional requirements shape architecture.
- Insist on numbers rather than adjectives.
- Say how you source the numbers when nobody knows them.
Example answer
Functional is what it does, non functional is how well it has to do it. In practice the second group is the one that gets me into trouble if I am lazy about it, because they shape the architecture. If a report has to come back in under two seconds, or the system has to keep an audit trail for seven years, or it has to work with a screen reader, those decisions get made in the first few weeks or they cost a fortune later. So I chase numbers rather than adjectives. Fast is not a requirement; ninety fifth percentile under two seconds at a thousand concurrent users is. I usually have to go and find those numbers myself, because when I ask a business stakeholder how fast it needs to be, the answer is fast, and when I ask what the current system does, nobody knows. So I measure the existing one and use that to start a real conversation.
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See how it worksFollow-up questions to expect
- Which non functional requirements get forgotten most often in your experience?
- How do you test a non functional requirement during acceptance?
- Who signs off performance and security requirements?
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