Technical project managers are great at identifying and classifying project requirements, but sometimes they get so caught up in the functional requirement to make a solution work that they forget the nonfunctional requirement of how the solution works.
In Brief:
- Nonfunctional requirements determine whether your system works in practice.
- Establish defining requirements and performance standards early, not later.
- Test beyond functionality. Performance, load and failover testing during quality assurance (QA) reveals critical weaknesses that only surface under real-world stress conditions.
- Assign clear ownership. Like functional features, nonfunctional requirements need dedicated owners to track and ensure delivery.
- Help stakeholders understand that speed, stability, and scalability make features usable and sustainable over time, not simply deployable.
When a project fails, it’s rarely because the team didn’t try hard enough. More often, the failure stems from something subtler — something you might not notice until it’s too late. You gather user needs, define features, get signoff, and launch. But then the system runs slowly. It crashes under pressure. Or users can’t rely on it when they need it most.
That’s not a failure of functionality. That’s a failure of nonfunctional requirements: the “how” behind the “what.” Neglecting nonfunctional requirements like performance, scalability and reliability can lead to systems that, while functionally complete, are unusable in practice.
Years ago, I (John) saw this firsthand while implementing an enterprise resource planning (ERP) system for a casino chain in Las Vegas. On paper, the system met every functional need. It handled purchasing, inventory, general ledger, and accounts payable. But when the 24/7 restaurants tried to track inventory in real time, the system couldn’t keep up. It wasn’t a bug. It wasn’t a logic error. It was a nonfunctional failure: The system simply couldn’t process transactions fast enough to stay current.
If you’re not building for performance, availability and scalability from the start, your system might do everything it’s supposed to — just not in a way that actually works for the people who rely on it.
What’s the Difference Between a Functional Requirement and Nonfunctional Requirement?
Once you’ve outlined a system’s goals, the next step is defining what it should do and how it should perform. That’s where functional requirements and nonfunctional requirements come in — and both are essential to project success.
Back in that Las Vegas casino ERP project, my team nailed the functional side. The system could:
- Track inventory of food and alcohol
- Log incoming shipments
- Generate purchasing reports
That’s the “what.” And on paper, it looked like the solution was the right one.
But the system struggled once it went live in the restaurants. These weren’t typical 9-to-5 businesses. They operated 24/7 and received deliveries and updated inventory at all hours. What the team failed to define and plan for were the nonfunctional requirements: how the system needed to behave in a real-time, high-volume environment.
Here’s the contrast:
- Functional: Track inventory and generate reports
- Nonfunctional: Update inventory in real time and support 24/7 restaurant operations
The system worked, but not fast enough or reliably enough for users to trust it. In other words, it met the functional requirements but failed on the nonfunctional ones.
This isn’t simply anecdotal. According to a 2023 study conducted for a new book, “Impact Engineering,” projects that defined requirements clearly before development were 97 percent more likely to succeed than those that didn’t. And that includes both what the system should do and how it should behave once live.
Functional gaps are easy to catch during demos. But nonfunctional requirements, such as performance underload or round-the-clock uptime, often remain invisible until users start experiencing real issues. That’s when trust erodes and adoption falters.
If you only focus on the “what” and ignore the “how,” you risk building a system that checks every box — but still lets your users down.
Why Nonfunctional Requirements Get Overlooked
If you’ve been in project delivery long enough, you’ve probably seen this happen: everyone’s aligned around the big features, the flashy functionality, the user demos. And somewhere along the way, nonfunctional requirements fall off the radar.
It’s not that you or your team don’t care about performance or reliability. It’s that these requirements are harder to define, test and assign ownership for. They often get lumped into vague documentation or pushed to the “we’ll get to that later” phase.
There are a few key reasons this happens:
- Functional requirements feel more tangible. Functional requirements, such as “process transactions” or “generate reports,” are straightforward and measurable. They provide clear targets for development and testing. In contrast, nonfunctional requirements like “system should respond within two seconds under peak load” are less tangible and often lack immediate visibility during development phases.
- They often hide behind assumptions. Teams often assume that nonfunctional aspects will “just work,” especially when using established platforms or frameworks. In the casino ERP project, there was an assumption that the inventory system would handle the same pace across all business units. However, the 24/7 operations of the restaurants introduced a load that the system wasn’t prepared for, leading to performance issues.
- They’re frequently under-scoped or underfunded. Nonfunctional requirements are sometimes underestimated in terms of their impact on the overall system performance and user satisfaction. This underestimation can lead to inadequate planning and resource allocation.
- There’s no clear owner. While functional requirements usually have designated owners or stakeholders, nonfunctional requirements often fall into a gray area with no clear accountability. This lack of ownership can result in these requirements being neglected during the development process.
Neglecting nonfunctional requirements can have significant consequences. According to a 2023 report, poor requirements gathering is the leading cause of software project failures, cited in 39 percent of cases.
But it doesn’t have to be that way. If you can bring the same clarity and discipline to nonfunctional requirements that you apply to functional ones, you can catch issues earlier, improve system performance, and prevent costly rework. Here’s how to make sure nonfunctional requirements don’t get lost in the shuffle.
How to Keep Nonfunctional Requirements from Derailing Your Project
You don’t need to overhaul your process to get nonfunctional requirements right. You simply need to bring them into focus earlier and treat them with the same importance as the functionality you’re building.
Here are some best practices about how to do that effectively:
1. Start With the Environment, Not Only the User
When you define requirements, you’re probably thinking about user requirements: what they need to do, what screens they’ll use, and what data they’ll need. But nonfunctional requirements depend just as much on “where” and “when” the system is being used.
In the casino project, the failure didn’t come from users asking for the wrong thing — it came from not accounting for the 24/7 operating environment. That reality should have driven requirements for real-time data updates, high availability, and performance at scale.
As a colleague of mine puts it: “Unless you define the explicit, you have to live with the implicit.”
Before you finalize your system requirements, ask:
- Will this system run around the clock?
- How many users will access it concurrently?
- Are there business units with unique performance demands?
- What security requirements will there be (such as multifactor authentication or password expiration)?
This ensures you’re building for context — not just capability.
2. Define Nonfunctional Requirements as Early as Possible
Don’t treat nonfunctional requirements as something to bolt on later. Start capturing them during discovery and planning. When you create user stories or feature specs, include constraints like:
- Max response time under peak load
- Required system uptime (such as 99.95 percent)
- Data processing limits (for example, transactions per second)
Make these measurable. “The system should be fast” isn’t a requirement — it’s a hope. “Pages must load in under two seconds 95 percent of the time during business hours” is a requirement.
3. Build Them Into Testing and QA
Functional testing gets all the attention, but performance, load and failover testing during QA are where many hidden problems emerge. That includes issues related to scalability, latency or timeouts that only show up under stress.
You don’t always need complex test scripts to validate nonfunctional requirements. Even something as simple as simulating a continuous stream of restaurant orders over a 24-hour cycle (as the team should have done in the casino example) can reveal critical weaknesses early.
4. Assign Ownership
Don’t let nonfunctional requirements float without accountability. Assign clear owners — whether it’s a tech lead, architect or QA manager. You’re more likely to hit your targets when someone is actively tracking them.
If your team uses objectives and key results (OKRs), consider assigning nonfunctional metrics as part of those key results. This reinforces their role in successful delivery, not just successful deployment.
5. Educate Stakeholders on Their Importance
Often, stakeholders push for visible progress: features that demo well, dashboards that “show results.” But speed, stability and scalability are what make those features usable and sustainable over time.
Use stories from past projects, including failures, to show why nonfunctional requirements matter. If a system crashes during peak hours or takes 30 seconds to generate a report, users won’t care how many features it offers.
When you treat nonfunctional requirements as essential, not optional, you reduce the risk of costly post-launch fixes and build a system that truly meets business needs. But none of this happens by accident. It takes deliberate effort to elevate these requirements from background noise to core delivery criteria.
And that effort starts with you.
Deliver More Than Just Features
As a project manager, your job isn’t only to get the system live. It’s to make sure it works in the real world. That means going beyond functional specs and digging into how the system will perform under real conditions.
When nonfunctional requirements get ignored, systems fail — not because they’re broken, but because they weren’t built for how people actually use them. So don’t let these details fall through the cracks. Build them into your planning, assign clear ownership, and make them part of how you define success.
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