A Guide to Functional Requirements (with Examples)

Learn how to define requirements and keep all stakeholders aligned.

Functional requirements

Getting the requirements right is the key to the success of any project. Failure to accurately define and document them inevitably results in miscommunication between stakeholders, constant revisions, and unnecessary delays. Studies show that unclear or poorly documented requirements can increase the project timeline and budget by up to 60%.

With the growing popularity of the Agile approach to documentation, some teams have started to neglect documenting requirements – after all, it's "working software over comprehensive documentation", right?

Alas, it's a common misconception, and foregoing proper documentation can be particularly damaging when it comes to requirements. In this article, we'll dive deeper into what functional requirements are and why it's vital to document them.

What are functional requirements?

Functional requirements are product features that developers must implement to enable the users to achieve their goals. They define the basic system behavior under specific conditions.

Business requirements, user requirements, product requirements, functional requirements, non-functional requirements

Functional requirements should not be confused with other types of requirements:

Functional requirements may be captured as part of a product requirements document (PRD) or in the form of a separate functional requirements document (FRD). Here's an example of what such a document may look like in Nuclino, a collaborative documentation tool for teams – create a free account and start documenting your requirements in one central place:

Functional requirements document in Nuclino

Why functional requirements need to be documented

Documenting and aligning on functional requirements has numerous benefits:

Functional requirements examples

Functional requirements need to be clear, simple, and unambiguous. Here are some examples of well-written functional requirements:

Contrary to a popular misconception, functional requirements are not analogous to user stories, but stories can be a useful tool for deriving requirements with the user in mind. For example:

Functional vs. non-functional requirements

When capturing product requirements, it's important to distinguish between functional and non-functional requirements.

To put it simply, functional requirements describe what the product should do, while non-functional requirements place constraints on how the product should do it. They can be expressed in the following form:

Functional requirements – as the name implies – refer to specific product functionality. Defining, measuring, and testing them is usually a straightforward task.

On the other hand, non-functional requirements (also known as "quality requirements" or "quality attributes") are more abstract. They impose constraints on the implementation of the functional requirements in terms of performance, security, reliability, scalability, portability, and so on.

NFRs are not themselves backlog items, but they are just as important since they ensure the usability and effectiveness of the entire software system. A transaction that takes 20 seconds to successfully complete may be functional – but it's certainly not usable.

Every functional requirement typically has a set of related non-functional requirements, for example:

How to write a functional requirements document

There is no universally accepted functional requirements document template, and it's up to you and your team which style and format to follow. However, there are several best practices that apply in most cases.

Select the right documentation tool

In the past, most teams used Microsoft Word to create and manage functional requirements. This inevitably led to out-of-date, inaccurate FRDs bouncing around the team's inboxes.

Fortunately, now you have more options to choose from. Select a tool that facilitates collaboration and ensures that everyone always has the latest version to avoid confusion. For example, you could store your requirements in a Google Doc, or better, in your team's internal wiki.

Make it a collaborative process

Your FRD needs to be a living document, evolving as your project progresses. To ensure that everyone stays on the same page, every stakeholder needs to continuously contribute. Involve your team early on and collaboratively keep the requirements up-to-date.

Be as clear as possible

Well-written functional requirements typically have the following characteristics:

Unclear or confusing requirements can create as many problems as undocumented ones. The scope of the project becomes fuzzy, leading to missed deadlines, unforeseen costs, and wasted effort. Making sure the requirements are documented in a way that leaves no room for interpretation can help you avoid these and many other issues down the road.

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