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Functional vs Non-functional Requirements

Functional vs Non-functional Requirements

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In the world of software development and project management, understanding the distinction between functional and non-functional requirements is crucial. These two categories of requirements play a pivotal role in shaping the success of a project. In this article, we will delve into the key differences between functional and non-functional requirements, their significance, and how they influence the development process.

What are Functional Requirements?

What Are Functional Requirements?

Functional requirements, often abbreviated as FRs, represent the core functionalities or features that a software system must possess to meet its intended purpose. In simpler terms, these requirements specify what the system should do. They describe the interactions between the software and its users, as well as the software’s behavior under various conditions.

Characteristics of Functional Requirements

Functional requirements typically possess the following characteristics:

  • Specificity: They are detailed and specific, leaving little room for ambiguity. They outline the precise functions, inputs, and outputs of the system.
  • Verifiability: Functional requirements are testable and can be validated to ensure that the software performs as expected.
  • User-Centric: They are closely aligned with the user’s needs and expectations, ensuring that the software fulfills its intended purpose.
  • Changeable: Functional requirements can change over the course of a project as user feedback and business needs evolve.

Examples of Functional Requirements

To better understand functional requirements, let’s consider a few examples:

  • User Authentication: The system must allow users to create accounts, log in, and reset their passwords.
  • Shopping Cart: The system must enable users to add items to their shopping cart, view the cart contents, and proceed to checkout.
  • Search Functionality: The system must provide a search feature that allows users to find products based on keywords.

Functional requirements form the foundation of a software project and guide the development team in building the desired features.

What are Non-Functional Requirements?

What Are Non-Functional Requirements?

Non-functional requirements, often abbreviated as NFRs, complement functional requirements by specifying how a software system should perform certain functions. They define the system’s qualities, characteristics, and constraints rather than its specific features. In essence, non-functional requirements set the standards for the system’s performance, security, and usability.

Characteristics of Non-Functional Requirements

Non-functional requirements exhibit the following characteristics:

  • Qualitative: Unlike functional requirements, which are typically quantitative, non-functional requirements focus on qualitative aspects such as performance, reliability, and security.
  • Global: Non-functional requirements apply to the entire system and impact its overall behavior.
  • Stability: They are generally more stable throughout a project’s lifecycle, with changes being less frequent compared to functional requirements.
  • Measurable: While non-functional requirements may be challenging to quantify precisely, they can still be measured and tested.

Examples of Non-Functional Requirements

Here are some common examples of non-functional requirements:

  • Performance: The system should load a webpage in under 3 seconds, even with 100 concurrent users.
  • Security: The system must comply with industry-specific security standards and encryption protocols.
  • Scalability: The application should be able to handle a 50% increase in user traffic within six months without degradation in performance.

Non-functional requirements ensure that the software operates efficiently and meets user expectations in terms of performance, security, and other critical aspects.

How are Functional Requirements Different From Non-functional Requirements?

Functional Requirements, as the name suggests, describe the functions of the system to be designed. It is a description of what the system will be and how it will function to satisfy user needs. They provide a clear description of how the system is supposed to respond to a particular command, the features, and what the users expect. 

Non-functional Requirements explain the limitations and constraints of the system to be designed. These requirements do not have any impact on the functionality of the application. Furthermore, there is a common practice of sub-classifying the non-functional requirements into various categories like:

  • User Interface
  • Reliability 
  • Security
  • Performance
  • Maintenance
  • Standards 

Sub-classifying the non-functional requirements is a good practice. It helps when creating a checklist of the requirements that are to be met in the system to be designed. 

Non-functional requirements are as important as functional requirements are. If functional requirements specify what a system should do, non-functional requirements describe how it will do it. For example, the new application shall provide us with the final list of all connected users. That is a part of functional requirements. If the requirement says that the system would only work on a Windows and a Linux system, that would be a part of non-functional requirements. 

The only difference between the two is that the system can not function without satisfying all the functional requirements. On the other hand, the system will give you the desired outcome even when it does not satisfy the non-functional requirements.

Key Differences and Collaboration

Understanding the differences between functional and non-functional requirements is essential for project success. While functional requirements define what a system should do, non-functional requirements specify how it should do it. These two types of requirements are complementary and should be developed in parallel to ensure a comprehensive understanding of the project’s scope and objectives.

Collaboration between business analysts, developers, testers, and stakeholders is crucial in eliciting, documenting, and prioritizing both types of requirements. The synergy between functional and non-functional requirements ensures that the final product aligns with user expectations while meeting performance, security, and other critical criteria.


Functional requirements typically describe what a system should do, while non-functional requirements place restrictions on how the system functions. When gathering requirements for a project, it’s important to consider both types in order to create a comprehensive list that will serve as the foundation for your development efforts. The Visure Requirements ALM Platform is a great tool for managing and traceability of both functional and non-functional requirements throughout the software development lifecycle. If you want to learn more about writing effective requirements, try the Requirements Specification Course by Visure Requirements ALM Platform today! 

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