What is Non-Functional Requirements?
Non-functional requirements (NFRs) specify how a system should perform rather than what it should do. They cover qualities such as performance, security, reliability, scalability, accessibility and maintainability, defining the measurable constraints and standards a product must satisfy alongside its functional features in order to be considered fit for use.
Reviewed by Gensudo Team · 23 July 2026
In more depth
Where functional requirements describe behaviours a user can trigger, NFRs describe the properties the whole system must hold, often expressed as thresholds such as "pages load in under two seconds at the 95th percentile." They are easy to omit because no single feature depends on them, yet they frequently determine whether a product is usable, trustworthy and legal. Many belong in the Definition of Done rather than individual stories.
Why it matters
Products that meet every functional requirement can still fail if they are slow, insecure, inaccessible or fall over under load. Making NFRs explicit and measurable turns vague expectations like "it should be fast" into testable targets, prevents late and expensive rework, and ensures quality attributes are designed in from the start rather than bolted on after launch.
A product example
A PRD for a checkout flow lists the functional steps, then the NFRs: 99.95% uptime, PCI-DSS compliance, WCAG 2.2 AA accessibility, and sub-300ms responses under 10,000 concurrent users. These constraints shape the architecture as much as the features do.
Documents where this shows up
Product Requirements Document (PRD) for Product Managers
Related terms
Product Requirements Document (PRD) · Acceptance Criteria · Definition of Done · Technical Debt