Functional vs non-functional requirements in practice
Functional requirements describe what the system should do – features and flows. Non-functional requirements describe how well it should do it: performance, security, availability, and usability. It's usually the non-functional requirements that get forgotten and turn into costly surprises late in the project. Make them measurable and prioritize them hard, because not everything can be most important.
Most requirements lists are full of what the system should do and nearly empty of how well it should do it. Right there, in what goes unsaid, hide the most expensive surprises. A system can meet every functional requirement and still be unusable because it’s too slow, too insecure, or collapses under load. This guide sorts out the difference, shows how to make the forgotten requirements measurable, and how to prioritize when everything feels equally important.
The difference in plain terms
Functional requirements describe the system’s features – what a user should be able to do. “The customer should be able to pay by card,” “the administrator should be able to export a report,” “the order should be sent to the warehouse.” They’re concrete and easy to come up with, since they match what people picture when they think about the system.
Non-functional requirements describe the qualities – how well the features should work. How fast the payment should go through, how many people can shop at once, how sensitive data is protected, how often the system is allowed to be down. They don’t show up in a feature list but decide whether the system holds up in reality. A common misunderstanding is thinking they’re “technical details” developers will handle anyway. In fact, they’re business requirements: a system that’s too slow loses customers, an insecure system becomes a liability.
A catalog of the forgotten requirement areas
Precisely because non-functional requirements feel self-evident, they often go unsaid. Here are the areas that are most often missing, with examples of how a requirement might be phrased:
- Performance. “A page should load in under 2 seconds for 95 percent of requests with 500 concurrent users.”
- Scalability. “The system should handle a tenfold increase in users without re-architecture.”
- Availability and uptime. “The service should be available 99.9 percent of the time per month, measured outside planned windows.”
- Security. “All personal data should be encrypted at rest and in transit, and access should be logged.”
- Usability. “A new user should be able to complete an order without instructions.”
- Accessibility for everyone. “The interface should meet WCAG level AA.”
- Maintainability. “The system should be documented so a new developer can get up to speed in a reasonable time.”
The list isn’t complete, but it captures the areas whose absence tends to be most costly. Just going through it with your project often reveals requirements that would otherwise have surfaced first in production.
How to make requirements measurable and testable
A non-functional requirement that can’t be measured is worthless, since no one can say whether it’s met. “The system should be fast and secure” sounds good but means nothing – it can’t be tested and therefore can’t be required.
The difference lies in swapping the feeling for a number, a condition, and a way to verify it. “Fast” becomes “under 2 seconds with 500 concurrent users.” “Secure” becomes concrete requirements for encryption, permissions, and logging. “Stable” becomes a figure for allowed downtime. Once the requirement is phrased that way, you can build against it, test against it, and hold the vendor accountable for it. A good test for a requirement is simple: could two people disagree about whether it’s met? Then it’s not measurable yet.
Prioritizing when everything feels important
Once the requirements are written down, the next trap appears: everything feels like a must. But if everything is top priority, there’s no prioritization at all, and then chance or whoever shouts loudest decides.
A proven method is to sort each requirement into must, should, and could – with a deliberate cap on how many can be “must.” That forces the real trade-offs into the open. A complementary approach is to ask, for each requirement: what happens if it isn’t met? If the business can live with the consequence, the requirement is rarely a must, however desirable it may be.
| Priority | Meaning |
|---|---|
| Must | The system is unusable or illegal without it – the cap is kept deliberately low |
| Should | Delivers clear value but can be pushed to a later version |
| Could | Nice to have if time and budget allow, but never at the expense of a must |
The point is that the prioritizing happens on paper, calmly, instead of under pressure mid-project when a forgotten requirement suddenly collides with the budget.
Take requirements seriously early
Functional requirements are easy to come up with and rarely become the project’s pitfall. It’s the non-functional ones – performance, security, operations, maintenance – that determine whether the system holds up in reality, and they cost a fraction to build in from the start compared with adding them later.
At Weapp we often help buyers develop exactly that requirements picture as part of our services, often in a short discovery phase before the build begins. Facing a project where the requirements need sharpening? Get in touch and we’ll talk through which non-functional requirements are worth pinning down first.
Frequently asked questions
What's the difference between functional and non-functional requirements?
Functional requirements describe what the system should do: a user should be able to log in, place an order, generate a report. Non-functional requirements describe how well that should happen: how fast, how securely, for how many simultaneous users. Functional requirements are about features, non-functional ones about qualities. Both are needed, but it's the latter that's most often forgotten.
Why do missing non-functional requirements get so expensive?
Because they affect how the entire system is built, not just a single feature. Discover late that the system must handle ten times more users, or meet a security requirement, and it can mean the foundation has to be redone. A performance requirement known from the start shapes the architecture cheaply; the same requirement surfacing after launch can mean a rebuild.
How do you make a non-functional requirement measurable?
By replacing vague words with numbers and conditions. "The system should be fast" can't be tested. "A page should load in under two seconds for 95 percent of requests with 500 concurrent users" can be measured and specified. A measurable requirement has a number, a condition, and a way to verify it – otherwise it's just a hope.
Which non-functional areas get forgotten most often?
Performance under load, security, availability and uptime, and what happens when something fails. Also scalability, accessibility for people with disabilities, and how easy the system is to maintain often fall outside the requirements picture. They feel self-evident and so go unsaid, until they're missing from the finished system and have to be built in afterward.
How do you prioritize when every requirement feels important?
By forcing a distinction instead of listing everything as important. One simple method is to sort requirements into must, should, and could – and set a cap on how many can be "must." Another approach is to ask what happens if the requirement isn't met. If the business can live with it, it's rarely a must.