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Functional vs Non-functional Requirements

Functional vs Non-functional Requirements

Table of Contents

What are Functional Requirements?

A functional requirement is a statement of how a system must behave. It defines what the system should do in order to meet the user’s needs or expectations. Functional requirements can be thought of as features that the user detects. They are different from non-functional requirements, which define how the system should work internally (e.g., performance, security, etc.).

Functional requirements are made up of two parts: function and behavior. The function is what the system does (e.g., “calculate sales tax”). The behavior is how the system does it (e.g., “The system shall calculate the sales tax by multiplying the purchase price by the tax rate.”).

What are Non-Functional Requirements?

Non-Functional Requirements (NFRs) are the constraints or the requirements imposed on the system. They specify the quality attribute of the software. Non-Functional Requirements deal with issues like scalability, maintainability, performance, portability, security, reliability, and many more. Non-Functional Requirements address vital issues of quality for software systems.

Categories of Non-Functional Requirements

Non-Functional Requirements can be divided into two categories:

  1. Quality Attributes: These are the characteristics of the system that determine its overall quality. Examples of quality attributes include security, performance, and usability.
  2. Constraints: These are the limitations imposed on the system. Examples of constraints include time, resources, and environment.

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.


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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