Understanding how functional and non-functional requirements relate and why both matter in software systems

Let me ask you a quick, down-to-earth question: when you think about what a software or system should do, what comes first—the things it must perform or the vibe it should deliver? The answer isn’t as dramatic as it sounds; it’s just a clean split in the requirements world. All requirements can be grouped into two buckets: functional and non-functional. That simple division helps everyone—from product folks to developers—to be on the same page about what a system does and how well it does it.

Let’s unpack what that means, and why this distinction is more than a nerdy classification.

The two buckets in plain terms

• Functional requirements: these are the “what” of the system. They describe behaviors, actions, and features. If you push a button, what happens? If a user logs in, what data is checked? If a report is requested, what content appears? Functional requirements spell out the concrete tasks the system should perform.

Examples you’ve probably seen:

• The app shall allow a user to create an account.

• The search feature shall return relevant results within three seconds.

• The checkout process shall support three payment methods.

• Non-functional requirements: these are the “how well” attributes—how the system behaves while doing those things. They define the quality criteria that judge operation, not the specific actions themselves. Think performance, security, usability, reliability, and maintainability.

Examples you’ve likely encountered:

• The login page shall resist brute-force attacks and lock after five failed attempts.

• The website should load the home page in under two seconds on a standard connection.

• The interface shall be accessible according to WCAG 2.1 guidelines.

• The system shall recover from server failures within 30 seconds.

It’s tempting to think one category is more important than the other, or that non-functional stuff is just a nicer add-on. But that’s a misconception. Both kinds of requirements govern a system’s success. Functional requirements tell you what you’re building; non-functional requirements tell you how good it will feel to use, how secure it will be, and how reliably it will run under pressure.

What about the other answer choices? A quick myth-busting

• B. Functional requirements are less important than non-functional requirements.

This one sells both sides short. They’re equally important. If a site can’t process a payment, it doesn’t matter how fast it loads. If a system is fast but insecure, users and stakeholders lose trust quickly.

• C. Non-functional requirements are simply an extension of functional requirements.

No—non-functional requirements aren’t a mere add-on. They’re separate quality criteria that shape the user experience and the system’s long-term viability. They don’t just “keep going” after the features exist; they define the conditions under which those features operate.

• D. Functional requirements address how something will be achieved.

Here, the wording trips people up. That phrase sounds like it’s about the implementation steps (the “how”). Functional requirements focus on what the system should do, not the specific implementation path. The “how” touches non-functional aspects like performance, security, and maintainability.

Putting both buckets to work in real projects

Why this split is not just a theoretical exercise matters in practice. It keeps teams honest about scope and quality. Here’s how the two types play nicely together in a real project, say, building a user-facing platform.

• Start with the user story or use case. What does a user expect to accomplish? That’s where functional requirements usually start. “A user should be able to reset their password.” The steps and data involved are part of the function.

• Parallel track the quality attributes. While the main flow is being outlined, you sketch non-functional targets. “Password reset must occur within 2 seconds; the reset link expires in 24 hours; the process must be auditable.” These targets set the bar for performance, security, and reliability.

• Use concrete acceptance criteria. Each functional item should have clear success criteria, and each non-functional attribute should have measurable thresholds. It’s not enough to say “fast.” You specify “under 2 seconds for the home page on a 5 Mbps connection.” That clarity helps testers, developers, and product owners stay aligned.

• Traceability matters. A good requirements set links each function to the business goal and to the relevant quality attributes. If a feature is in scope but has weak security requirements, the traceability helps surface that gap early.

A concrete example to anchor the idea

Imagine you’re in charge of a shopping app. A functional requirement might be: “The app shall allow a user to add items to a shopping cart.” That’s essential, yes. But you also need non-functional targets: “The cart should reflect updates within two seconds even during sales spikes,” and “The checkout process should be secure, with encryption in transit and at rest.” Without the non-functional targets, you could have a slick feature that users barely trust because it’s slow or risky. Without the functional target, you’d have a feature that sits on a shelf—nice to have, but nowhere near usable.

Quality attributes you’ll keep an eye on

If you want a compact mental map, think of these common non-functional categories (they’re the kinds you’ll see in standards and templates):

• Performance efficiency: response times, throughput, resource usage

• Usability: how easy the system is to learn and use

• Reliability: uptime, failure recovery, fault tolerance

• Security: authentication, authorization, data protection

• Maintainability: how easy it is to fix and modify

• Portability: how easily the system can run in different environments

• Compatibility: how well it plays with other systems and standards

You don’t need to memorize every label, but you’ll benefit from recognizing that these criteria shape decisions long before code is written.

A practical way to capture both kinds of requirements

Here’s a straightforward, human-friendly approach you can apply in teams of any size:

• Gather stories or use cases first. Focus on user intent and the critical flows.

• For each story, write one or more functional requirements describing the behavior.

• For the same stories, jot down the non-functional targets that matter to stakeholders (speed, security, accessibility, etc.).

• Prioritize with a simple method like MoSCoW (Must have, Should have, Could have, Won’t have this time) or a risk-based approach. Non-functional targets often becomeMust haves because users notice them immediately under load or in risky situations.

• Create a lightweight verification plan. Define how you’ll test both the function and the quality criteria. A test plan that checks “can login?” and “does login stay secure under load?” covers both halves.

A tiny digression about how people talk about quality

In the real world, non-functional requirements sometimes get labeled as “nice-to-have,” especially when a project is tight on time. Don’t let that lure you into underestimating their impact. Think of quality attributes as the weather on a road trip: you might not notice them when the sun is shining, but they matter a lot when a storm hits. A robust system is built to weather those storms without drama.

How this framing helps students and teammates

For those studying the field, framing requirements as two buckets is a clean mental model. It keeps conversations precise. It helps you avoid generic statements like “the system should be good.” Instead, you’ll talk about measurable speeds, safeguards, and reliability targets. And when you pair functional and non-functional requirements, you create a more complete blueprint that guides design, implementation, and testing.

A few quick notes you’ll find useful

• ISO and standard bodies often formalize quality attributes in a structured way. You’ll come across quality models that break down attributes into groups (for example, usability alongside accessibility, or security alongside privacy). Don’t feel you must memorize every label; focus on understanding the intent.

• Documentation can stay concise. The goal isn’t to produce a novel, but to have clear, testable statements. Short, precise sentences with measurable criteria beat long, vague descriptions every time.

• Stakeholders will appreciate when you translate ideas into concrete criteria. If a business leader asks for “high performance,” you’ll be ready to specify what that means in seconds or milliseconds. If a user worries about security, you’ll show concrete controls and how you’ll demonstrate them.

Bringing it home

Here’s the essence in a nutshell: every requirement you capture belongs to one of two worlds—functional or non-functional. Functional requirements tell the system what to do; non-functional requirements tell the system how well it should do it. They aren’t competing claims; they’re two halves of a single, coherent picture. When you document both clearly, you pave the way for a product that not only works but feels dependable, fast, and trustworthy.

If you’re ever tempted to skip the non-functional part because it sounds abstract or “extra,” pause. Ask a simple question: how would users describe the experience if everything worked but nothing felt reliable or secure? The answer will push you back to the truth: both kinds of requirements are essential, and they belong in every solid specification.

And that’s a mindset worth carrying—from the first sketch to the final release. A well-balanced set of requirements is like a well-tuned instrument: you hear the melody of what it does, and you feel the harmony of how it performs under pressure. Now you’ve got a practical framework to talk through both with clarity, whether you’re leading a team, reviewing a proposal, or just sharpening your own understanding of the craft.