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Agile Project management Technology

Functional vs non-functional requirements

For software development, outlining clear and detailed specifications based on the end-users’ needs and expectations is essential. At the planning stage, developers can use the requirements to decide how they should build the product so that future users will enjoy it. The requirement guidelines will also aid project managers in dividing work, setting project priorities, and defining the budget and implementation schedule.



Once the project begins, the requirements can be used to guide the developers throughout the entire development process and keep them from getting sidetracked.

We can distinguish two types of requirements: functional and non-functional. What are they, how are they different, and how can they help your team deliver successful projects?

This article will cover everything you need to know about them.

What are functional requirements?

Functional requirements (FRs) are specific features or functionalities that a ready product must have to allow its users to complete specific tasks. FRs are key to a project’s success since they define how useful the product will be for its target users.

FRs are made up of two parts: function and behavior. The function is what the system does (e.g., “Allow users to create an account”), while behavior focuses on how the system does it (e.g., “The system allows the users to create an account by filling in a form or by using their email or social media accounts.”)

So in simple terms, the job of FRs is to describe what the product must be able to do and how it should respond to user input.

Some of the most common types of FRs are:

  • User authentication and authorization levels,
  • Search features,
  • Data gathering, reporting, and analysis,
  • Legal or regulatory compliance needs,
  • Backup and recovery features,
  • Transaction processing, etc.

How can functional requirements be stored?

Depending on your team’s preference, functional requirements can be stored in many different ways. There’s no “right” or “wrong” method here, so you can use whichever method is most convenient for your team to document FRs.

What is crucial, however, is to clearly describe the specifications so that both the development team and the stakeholders understand how the ready product should behave and what the main project goals are.

The following are some of the most common formats and documents used to store functional requirements:

➡️ User stories

A user story is an informal, general explanation of a software feature written from the end user’s perspective. Basically, they are supposed to describe what goals the user has and what they want the system to do. User stories are generally stored inside the Agile project Backlog.

Compared to use cases that are often very detailed, user stories are much shorter (typically described in one sentence) and don’t include any technical details.


A user story must include acceptance criteria that indicate what conditions the product must meet to be accepted by the user, stakeholders, or product owner.

➡️ Use cases

The second option available here is use cases, which describe in detail how a user can interact with a system or product and how the system should respond to the actions. For example, if the use case is “Purchase a product”, the file should describe each step that users must take to complete the purchase.

Use cases typically outline scenarios in which the user successfully completes the task and the most crucial alternative paths or issues they might encounter.

Depending on which option would be more convenient for your team, use cases can be written in plain text or you can visualize them using diagrams.

➡️ Work breakdown structure

For more complex functional requirements, a work breakdown structure (WBS) is commonly used. A WBS is a visual document that helps to break down the main functionality into smaller components (processes, task, and subtasks) and thus allows development teams to better understand the project scope and the relationship between tasks.

For this method, it’s recommended that the features be broken down into smaller tasks until they can’t be any further.

➡️ Functional requirements document

A functional requirements document (FRD) is a document that developers can use to describe a product’s intended capabilities, functionality, appearance, and behavior in detail. The file can then serve as guidelines for teams during sprints.

An FRD can be a single document explaining the functional requirements to teams or can be used alongside other documents such as user stories and use cases.

What are non-functional requirements?

Non-functional requirements (NFRs) are a bit more challenging for development teams to describe and implement. They are not related to the system functionality, but instead define how the system should perform.

As such, NFRs are connected to product aspects such as:

  • Performance,
  • Security,
  • Reliability,
  • Scalability,
  • Portability, etc.

We talked a bit more about the main types of non-functional requirements in our previous article, so we won’t get into details here. However, we need to emphasize that although NFRs can be a bit trickier for Agile teams to choose, document, implement and test, they are essential to building a product that will be both functional and user-friendly.

While a website or application might still work without including the NFRs, it might later turn out that the users aren’t happy with its performance – for example, because the application crashes during peak load.

It can be pretty difficult to fix performance issues after a product has already been launched too, since parts may have to be rewritten to meet new performance or reliability goals. And that will, unfortunately, mean extra expenses on your side. Defining NFRs early on can help you avoid this.

To make the gathering and defining process easier, we shared a few tips on how you can implement non-functional requirements into your project in our previous article in the series.

How are both requirements types different?

Let’s look at an example to illustrate how important each type of requirement is.

Suppose you are building a new website, and you want to include automatic email confirmations whenever someone makes a purchase. The confirmation feature itself would be a functional requirement. If you want to specify that the emails should be sent ten seconds after the system notices the new purchase though, you will need to write this as a non-functional requirement.

Basically, FRs focus only on what a specific feature or functionality should do, while NFRs describe how a given feature should work.

There are, however, a few more differences between functional and non-functional requirements. You can see their main characteristics in the table below.

Describe what the product does
Describe how the product will work
GoalDefine product featuresDefine product properties (security, reliability, etc.)
Focuses on…Product functionalityQuality criteria
How they are storedIn functional requirement documents, use cases, user stories, or work breakdown structures.Can be stored in user stories, acceptance criteria, or technical stories.
Mandatory?Yes, the product won’t work properly without those.Not mandatory (product will work without those) but desirable
Types (examples)– User authentication and authorization levels,
– Search features,
– Data gathering, reporting and analysis,
– Legal or regulatory requirements, etc.
– Security
– Reliability
– Scalability
– Performance, etc
Capturing, documenting and testingEasier for development teamsHarder for development teams
What is testedWhat is tested
Components, API, UI, Integrations, etc.
Performance, security, usability, reliability, etc.

Why should you document the project requirements?

Gathering and documenting all project specifications (especially the non-functional ones) may sometimes feel tedious. Can a project succeed with vague goals or barely any requirement documentation at all?

That would be hard, if not impossible. In fact, project managers often cite inaccurate or vague requirements as one of the reasons why they and their teams are unable to complete a project on time.

Project requirements are basically the “why?”, “what?”, and “how?” of your project. The more precise and easy to understand they are, the easier it will be for your team and stakeholders to keep the project on track and then deliver a ready product successfully.

The following are a few ways that clearly defined requirements can make your project more successful.

Makes it easier to organize, prioritize, and divide the work between team members

Having all the project specifications documented makes it easy to see what needs to be done, what resources are required, and what the main goals of the project are. This makes it far simpler to decide on the order in which tasks should be completed, to set milestones and deadlines, and to pick the right developers for each task.

Keeps developers and stakeholders on the same page

A detailed project requirements document helps both parties understand what the main goals are and what their role in the project will be. Also, having all crucial details and steps outlined in documentation helps developers work more efficiently, as they won’t have to keep asking their teammates or project managers about every aspect of the project.

Allows for more accurate estimation of the required timeline and budget

Knowing what the main goals and needs are at each stage of the project will help you gauge how much time and resources they’ll need. If some of the noted requirements are complex, it might be a good idea to first break them down into smaller tasks and subtasks and only then estimate how much time your team will need to complete them.

Makes communication smoother

Documenting the project can help you track what was decided and agreed upon at each stage and what’s been done already. This is especially important when there are multiple teams or stakeholders involved, as it can be challenging to keep track of everyone’s input otherwise. That could lead to misunderstandings and unnecessary delays.

Prevents setbacks or technical issues during development

Using a detailed specification list can help your developers point out places where there might be delays or development issues and share their solutions to the problem. By doing so, your team can save plenty of time (and stress) during the later development stages, as they will know how to handle a given problem straight away rather than spend time searching for a solution and then rewriting parts of the product.


How do we at Inwedo work with project requirements?

After over 15 years of helping clients build their dream products, Agile projects are second nature to us. So, when it comes to analyzing and mapping out project specifications, planning sprint scopes, and consistently delivering working features, we are experts.

Of course, we know full well that requirements aren’t set in stone – rather, they should follow the changing needs and priorities of the project. That’s why we keep analyzing, discussing, and verifying the project requirements during regular sprint meetings to ensure that the final product meets all of the client’s needs.

Do you have a project idea you’d like the Inwedo team to take on? Just give us a call and we’ll schedule a time to speak about your goals.


Both functional and non-functional requirements are essential for digital projects to be successful. The former will help your team build a product with exactly the features your target audience expects, while the latter ensure that users have the best possible experience when they are using it. Focusing on both of these requirements allows you to build a product that will truly stand out, which is definitely worth the time and effort.

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