The Importance of Functional and Non-Functional Requirements for a Project

Strict requirements, drawn up by the development team, and recorded in a single document are an extremely important step in product development.

Distinguish between functional and non-functional requirements, each of which has its own meaning. What requirements apply to each of the types, and whether they follow

each other, you will learn from this article.

What Are Functional Requirements?

Any requirement that defines what the system should do is called a functional requirement. Functional requirements help capture the expectations of the system’s performance. Thus, these requirements must specify all aspects of the system’s behavior when certain conditions are met. A striking example of functional requirements is sending a letter when registering a new client in the system.

Thus, you can specify all the parameters of the function: the input parameter, the reaction to it, and the output. It can be not only about mailings, but also about data processing, business processes, and calculations.

Examples of functional requirements:

  • Corrections and cancellations of transactions
  • Authentication and Authorization Levels
  • Administrative functions and business rules
  • Saving the history of transactions
  • Legal or Regulatory Requirements.

Failure to comply with functional requirements leads to the fact that the system simply stops working. Thus, functional requirements represent the description features of an individual product, which are focused on the needs of users.

What Are Non-Functional Requirements?

If functional requirements indicate WHAT to do, non-functional ones explain HOW to do it. These kinds of requirements have no effect on the basic functionality of the system, and even if problems arise with them, the basic purpose of the system will be fulfilled without problems.

Non-functional requirements are important from a user experience perspective. They are responsible for interaction with the user and help determine the simplicity and performance of the system. Non-literal requirements are responsible for meeting the user’s expectations.

To understand the difference, imagine the page loading process. Functional requirements provide fast page loading, but non-functional requirements will determine how fast it should load.

A basic list of non-functional requirements may include:

  • Performance
  • Usability
  • Reliability.

That is, the entire opinion about the product will be based on non-functional requirements.

What Is the Difference Between Functional and Non-Functional Requirements?

As mentioned above, functional and non-functional requirements are related. They complement each other, creating not only an effective but also a convenient product.

Consider other significant differences between functional and non-functional requirements:

  • While functional requirements define a component or system as a whole, non-functional requirements indicate a performance attribute.
  • The functional requirements are the “verbs” of the system: management, remediation, tracking, auditing, cancellation, reporting. But for non-functional requirements, like attributes, include scalability, product availability, data recovery, and integrity work, etc.
  • Through requirements analysis, functional requirements can identify missing requirements for product improvement. Non-functional requirements will precisely set the level of requirements for product improvement and help direct them to improve the quality of service and satisfaction at the level of user experience.

How to Gather the Requirements?

The best solution, in this case, would be to brainstorm and/or interview potential users. They might be your team members who need this app.

Basic functional requirements to consider:

  • Business requirements determine not only the final product but even the authorization levels.
  • System requirements will determine the software and hardware specifications, as well as the specific responses of the system.
  • Administrative functions will describe the system, what reports to provide, and how to function in general.
  • User requirements and potential actions within the application or site.
  • Then move on to defining non-functional requirements:
  • User interface usability is a key factor in shaping the user’s opinion of your product.
  • Product scalability, product performance, availability, and safety in general. If it fails to meet the needs of the consumer, interest in such an application will below.

Most often, all requirements are worked out in parallel, without dividing into functional and non-functional. The classification can be done, for example, by business analysts or project managers.

How to Write Both Requirements?

The most popular way to write functional and non-functional requirements is like a document containing requirements specifications.

Read also: Best Practices on How to Write Quality Technical Documentation for Your Project

Such a document should set out the very essence of the project, its overview, as well as a list of limitations and hypotheses.

In addition to the textual description of the product, it is advisable to insert diagrams and screenshots into such a document, which will help in a deeper understanding of the details, even for those who do not have technical knowledge.

Work breakdown structure (WBS) is associated with such documentation. This allows you to break down the app development processes into smaller ones and facilitate the elaboration of details.

Also, all this can be presented in the form of so-called “user stories”. In essence, they are a description of the functional requirements from the perspective of the end-user and his clear expectations from the system. Sometimes user stories are used as the basis for writing specification documentation and SRS documents (or requirements impact software).

The Examples of Functional and Non-Functional Requirements

Let’s consider examples of functional and non-functional requirements.

Functional requirements

  • Approval from the system after registering a new user.
  • The server registers any changes to the data.
  • Creation of backup copies of data in automatic mode.
  • Automatic distribution of error reports.
  • A clear hierarchy in the system with specific data access parameters.

Non-functional requirement

  • Compliance with database security with HIPAA requirements.
  • Getting access to your account in three clicks.
  • Request to change the password if the user does not log in to the system for three months.
  • Page loading should take no more than 4 seconds.
  • The background color for all screens should be #fff99b6.
  • Server operation should be uninterrupted for 24 hours.

The Best Practices for Requirements

The key to effective functional and nonfunctional requirements in their simplicity and clarity. Explaining functions using an example of user action is considered the most preferred format for web-development teams.

Avoid vague explanations of what is happening, provide accurate data and details. Even the use of the word “necessary” instead of the more general and lacking specificity will help guide you in the right direction.

If you understand that there can be confusion in measurement systems or calculations, pay special attention to this and describe the standards separately.

Any described requirement can be checked and understood what should be the result. You must provide all the scenarios, describing the reactions of users and the system. You should always be able to track each requirement against one of the project goals. Do not forget about competitor analysis, analyze user cases and find weak points that you can refine and offer users the best solutions.

Make sure the specifications are consistent with each other and that they are truly feasible and workable in terms of budgets. Don’t underestimate non-functional requirements, think not only as a businessman but also as a regular user.


The separation of non-functional and functional requirements is a technical process that is an integral part of development. Writing specifications will not only help structure expectations but also clearly prioritize.

Requirements also define the long-term prospects of a product, defining its characteristics, audience expectations, and so on. Create the requirements comparison table for a better understanding of defining functional requirements and non-functional ones.




Web & mobile app development company that creates digital solutions for startups & businesses (native iOS, Android apps, outstaffing, outsourcing) @

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TopDevs Inc.

TopDevs Inc.

Web & mobile app development company that creates digital solutions for startups & businesses (native iOS, Android apps, outstaffing, outsourcing) @

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