Non-functional requirements describe how a system should behave and why that matters.

Outline ( skeleton )

• Hook: Many folks think requirements are just a list of features. In reality, non-functional requirements shape how the system feels and behaves.

• What non-functional requirements are (the “how” behind the “what”)

• How they differ from functional requirements (two sides of the same coin)

• Quality attributes that matter (performance, usability, reliability, security, maintainability, portability)

• Concrete examples you’ll recognize in real systems

• How to capture and verify them (stakeholders, constraints, measurable targets)

• Common missteps and practical tips

• Quick-start checklist for identifying non-functional needs

• Takeaway: these requirements aren’t optional frosting—they’re core to user trust and system robustness

Article: Non-functional requirements explain how the system should work

Let me explain a small but powerful idea that's easy to miss: non-functional requirements describe how the system should work, not just what it should do. You might picture a product as a list of features—login, search, report generation. Functional requirements cover those tasks. Non-functional requirements, on the other hand, set the standard for the experience around those tasks. They say things like: the system must respond quickly, be easy to use, stay reliable, and keep data safe. Think of them as the quality attributes that hold the functionality together.

What are non-functional requirements, exactly?

• They’re the “how” behind the “what.” Functional requirements answer the question “What should the system do?” Non-functional requirements answer questions like “How fast should it respond? How usable is it? How secure is it? How well does it behave under load?” In the IREB Foundation Level landscape, you’ll see these notions tied to quality attributes that software teams care about day to day.

• They’re not tasks. They’re constraints and targets that guide design, implementation, and testing. If a feature is a function, the non-functional requirements are the guardrails that keep that function usable in the real world.

How non-functional requirements sit beside functional ones

Imagine you’re building a web app that lets people book flights. A functional requirement might be: “The system shall allow users to search for flights by origin, destination, and date.” A non-functional counterpart could be: “The search results must appear within two seconds for 95% of requests.” Or consider usability: “The booking flow should be operable by a user with average computer literacy without external help.” Both kinds of requirements are essential. The functional part does the job; the non-functional part makes sure the job is done well enough to count as a good user experience.

Quality attributes you’ll frequently encounter

Here’s a pragmatic mix you’ll come across:

• Performance and responsiveness: How fast does the system react? Two-second responses are a common target for user flows, but you’ll see longer windows for heavier tasks. The key is to specify a measurable target and to test against it.

• Usability: Is the interface intuitive? Are there enough visual cues, sensible defaults, and accessible features? Usability isn’t a “nice to have”—it directly influences adoption and satisfaction.

• Reliability and availability: Will the system stay up? What’s the acceptable downtime, and how quickly should it come back after a mishap? Reliability also covers recovery behavior after failures.

• Security and privacy: How are data and identities protected? What measures guard against unauthorized access, and how is sensitive information handled?

• Maintainability: How easy is it to fix, modify, or extend the system? This often translates into code quality, documentation, and modular design.

• Portability and compatibility: Can the system run in different environments or integrate with existing tools and standards? This is especially important if the software must live in diverse tech stacks.

• Accessibility: Can people with disabilities use the system effectively? Accessibility isn’t optional; it broadens reach and reduces risk of exclusion.

Concrete examples to anchor the idea

• Performance example: “The system shall return search results within two seconds for 95% of queries under a typical load.” This creates a concrete target for developers and testers.

• Usability example: “Users should complete the checkout flow with no more than six clicks from landing page to order confirmation, and the interface must be self-explanatory for first-time visitors.” You’re speaking to the user’s journey, not just a function.

• Reliability example: “The service will maintain 99.9% uptime per calendar month, excluding scheduled maintenance, with automatic failover to a standby instance.” This sets expectations for operations teams and helps plan capacity.

• Security example: “All user passwords must be stored using salted hashing, and all sensitive data must be transmitted over TLS 1.2 or higher.” It’s specific, actionable, and testable.

• Maintainability example: “Code modules should have unit tests covering at least 80% of lines, and documentation must be updated with each release.” It signals the team’s willingness to adapt and improve.

• Portability example: “The application should run on Windows, macOS, and Linux, with no dependency on browser plugins.” It defines the environments where the system should operate smoothly.

How to capture non-functional requirements without sinking into theory

• Start with stakeholders’ expectations: talk to users, operators, and security folks. What would make their lives easier or harder? What risks keep them up at night?

• Translate vague needs into measurable targets: instead of “fast,” say “under two seconds.” Instead of “secure,” outline concrete controls and auditability.

• Tie requirements to the system’s quality model: reference a model like ISO 25010, which groups attributes into clear categories. It provides a common language that teams can rally around.

• Include constraints and boundaries: reflect realities like budget, hardware, and regulatory demands. Constraints aren’t limits—they’re part of the design.

• Prioritize with a practical lens: not every attribute can be perfect. Decide what matters most to users and business goals, and document the reasoning.

What often goes wrong (and how to fix it)

• Vague targets: If you say “fast enough,” you’ve left the door open for misinterpretation. Fix it with numbers, time bounds, and acceptance criteria.

• Mixing terms: Don’t confuse performance with throughput, or security with compliance. Be precise about what you mean in each context.

• Missing testing strategy: You can’t verify a non-functional requirement by accident. Plan tests, mocks, load scenarios, and monitoring that prove the target is met.

• Overloading requirements: It’s easy to pile on a lot of attributes. But when targets clash, you create a moving target that’s hard to satisfy. Focus on the most impactful ones first.

A practical, quick-start checklist

• Identify the high-impact quality attributes for your system (performance, usability, reliability, security, maintainability, portability, accessibility).

• For each attribute, state a measurable target (e.g., response time, uptime, error rate, accessibility conformance level).

• Link each target to a real-world scenario or user story so everyone understands the context.

• Define how you’ll test or demonstrate that the target is met (tests, monitoring, audits).

• Document any constraints that shape the target (legal requirements, hardware, budget).

• Review with stakeholders and adjust priorities as needed.

Bringing it together: why non-functional requirements matter

Here’s the practical takeaway: non-functional requirements aren’t abstract niceties. They shape real experiences. They determine whether a user sticks around, whether data stays safe, and whether a system continues to perform as needs evolve. They influence how teams architect the solution, how QA validates it, and how operators keep the lights on. When you articulate these requirements clearly, you give designers, developers, testers, and managers a shared compass. You reduce surprises and increase the chances of delivering something that feels solid in the wild.

A few reflective thoughts to end on

• The best products feel fast, safe, and forgiving. People notice when something is off long before they can articulate why. Non-functional requirements help you catch that intuition in concrete terms.

• You don’t have to chase every attribute at once. Start with the pain points you’ve observed in real usage, then layer on additional quality targets as the system grows.

• In the end, it’s about trust. If users trust the system to behave predictably, they’ll return, recommend it, and rely on it when it matters most.

Final takeaway

Non-functional requirements describe how the system should work. They set the pace, the mood, and the safety net for the user experience. They’re the measurable, testable engine behind every smooth interaction. If you keep them in view as you design and build, you’re less likely to end up with a feature that’s technically correct but emotionally underwhelming.

If you’d like, I can tailor a short set of non-functional targets for a specific domain—whether it’s e-commerce, finance, or healthcare—so you see how these ideas play out in a real system.