What Are Non-Functional Requirements? Definition + Best Practices

Non-functional requirements are global constraints on a software system e.g., development costs, operational costs, performance, reliability, maintainability, portability, robustness etc.

A requirement that does not relate to functionality, but to attributes such as reliability, efficiency, usability, maintainability, and portability.

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.

Non-functional requirements may be found in adverbs or modifying clauses, such as “The system prints invoices quickly” or “The system prints invoices *with confidentiality.”

Software has non-functional requirements that are essential to creating a positive experience for the user. And if you miss the mark on these requirements, you may have scheduling delays, budget setbacks and unsatisfied clients.

A project that lacks non-functional requirements may also lack clarity, which can result in vague project scope and a disconnect between the designer and client expectations.

Non-functional requirements bridge the gap between what developers believe customers want and what customers really want.

One study found that 60 to 80 percent of the cost of software development involves rework. And when the non-functional requirements are done well, you may eliminate 50 to 80 percent of product defects.

So, now that we’ve defined non-functional requirements, how do you integrate them effectively into your product development process?