Functional vs non-functional requirements in software development 

Defining requirements is a critical aspect of software development. In fact, many software development projects fail simply because of the way in which requirements are outlined. A lack of clarity, precision, and feasibility can all lead to project disappointment. 

When it comes to software development requirements, there are two important categories, functional and non-functional requirements. To fully understand what we mean by defining requirements, we need to know exactly what one looks like and what form it can take. It is, therefore, crucial to understand these two categories and the differences between them.

In this blog, we are going to discuss this topic and explore it in an accessible fashion. Without full knowledge of requirements, we can never expect a project to succeed or to match client expectations. To make something we are proud of, we need to nail the requirements and know how to define them. 

Let’s start by clarifying some definitions. 

What are functional requirements?

Functional requirements in software development define what a system must do. They place constraints on the system’s functioning and establish critical rules of operation. These requirements specify behaviours and functions and they come in a wide variety. 

Some functional requirement examples include: 

  • Authentication factors in log-ins
  • Verification processes
  • Business rules 
  • Legal or GDPR requirements
  • System reporting requirements
  • External system interfaces 
  • System shutdowns (in case of cyber attacks)
  • Audit tracking 

When a user interacts with a system, functional requirements define what the system must do in return. They follow a basic “if X, then Y” formula. If a user attempts to log in with the right password and account, then the system authenticates the user and allows them to log into the system. If the wrong password is entered, access is blocked. This process is underpinned by functional requirements. 

You can think of functional requirements very much as setting the foundations of any system. 

What are non-functional requirements?

In comparison, non-functional requirements in software describe how the system should work, in a way that elaborates on the structure already laid down by functional requirements. The non-functional requirements are somewhat a top layer of requirements that allows the system to work in a way that benefits users. 

If a functional requirement outlines that a system should perform a certain function, non-functional requirements outline the ways in which it should do so. This might seem less important but this isn’t the case. Non-functional requirements are critical components of usability and performance. 

Where functional requirements are related to product features, their non-functional counterparts are related to product properties. 

Some areas covered by non-functional requirements include: 

  • Accessibility 
  • Speed
  • Availability (time constraints)
  • Maintenance protocols
  • Scalability 
  • Flexibility 
  • Performance
  • Security 
  • Testability 
  • Quality 

And many more. 

While all non-functional requirements may not be technically essential, they are crucial for meeting user expectations. If someone uses a system that does what they need it to do but it does so with very low response times with very long unavailable periods, this won’t be a system they enjoy or continue to use. 

To produce a piece of software that is widely used, enjoyed, and recommended, a software development company needs to carefully consider multiple non-functional requirements. This is where we would put ourselves in the user’s shoes and imagine what it would be like to navigate through the system first-hand. If it doesn’t flow or feel good, it is non-functional requirements we need to explore. 

Why does this matter? 

As we have touched on above, to produce the best possible quality in a piece of software, we need to carefully explore both functional and non-functional requirements. It is essential that both of these areas work in harmony to produce an end result that meets user and client expectations. If you have one without the other, the software will not be a complete package. 

Getting to grips with the different types of requirements and what they mean helps us to accurately and efficiently run a software project. Without the right emphasis on requirements, projects can easily fail. And that isn’t what we want. 

In 2022, we only want high-quality, high-performing software that works on all layers. Understanding requirements is simply one of the ways in which we can do so. 

Do you need a tech expert?


If your company has a project in mind and you are struggling to figure out requirements, planning and everything in between, why not hand everything over to tech experts at a software development company?


Software development is tricky and it requires the wearing of many expert hats all at once. Even something such as defining what a piece of software should do is complicated and nuanced, as we have seen above. Outsourcing your software needs to an expert software development company is the best way to get everything done in the right way. 


At 6B, we are a team of dedicated and experienced individuals who can work together to perfect your next project. 


You can leave everything in our capable hands.


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