What is a Functional Requirement in Software Engineering?

What is a Functional Requirement?

A Functional Requirement (FR) is a description of the service that the software must offer. It describes a software system or its component. A function is nothing but inputs to the software system, its behavior, and outputs. It can be a calculation, data manipulation, business process, user interaction, or any other specific functionality which defines what function a system is likely to perform. Functional Requirements in Software Engineering are also called Functional Specification.

In software engineering and systems engineering, a Functional Requirement can range from the high-level abstract statement of the sender’s necessity to detailed mathematical functional requirement specifications. Functional software requirements help you to capture the intended behaviour of the system.

What should be included in the Functional Requirements Document?

Here is how to write functional requirements document:

Example Functional Requirements
Example Functional Requirements

Functional Requirements of a system should include the following things:

  • Details of operations conducted in every screen
  • Data handling logic should be entered into the system
  • It should have descriptions of system reports or other outputs
  • Complete information about the workflows performed by the system
  • It should clearly define who will be allowed to create/modify/delete the data in the system
  • How the system will fulfill applicable regulatory and compliance needs should be captured in the functional document

Benefits of Functional Requirement

Here, are the pros/advantages of creating a typical functional requirement document-

  • Helps you to check whether the application is providing all the functionalities that were mentioned in the functional requirement of that application
  • A functional requirement document helps you to define the functionality of a system or one of its subsystems.
  • Functional requirements along with requirement analysis help identify missing requirements. They help clearly define the expected system service and behavior.
  • Errors caught in the Functional requirement gathering stage are the cheapest to fix.
  • Support user goals, tasks, or activities

Types of Functional Requirements

Here are the most common functional requirement types:

  • Transaction Handling
  • Business Rules
  • Certification Requirements
  • Reporting Requirements
  • Administrative functions
  • Authorization levels
  • Audit Tracking
  • External Interfaces
  • Historical Data management
  • Legal and Regulatory Requirements

Example of Functional Requirements

Below are the popular functional requirements examples:

  • The software automatically validates customers against the ABC Contact Management System
  • The Sales system should allow users to record customers sales
  • The background color for all windows in the application will be blue and have a hexadecimal RGB color value of 0x0000FF.
  • Only Managerial level employees have the right to view revenue data.
  • The software system should be integrated with banking API
  • The software system should pass Section 508 accessibility requirement.

Non Functional vs. Functional Requirements

Here, are key differences between Functional and Nonfunctional requirements in Software Engineering:

Parameters Functional Requirement Non-Functional Requirement
What it is Verb Attributes
Requirement It is mandatory It is non-mandatory
Capturing type It is captured in use case. It is captured as a quality attribute.
End result Product feature Product properties
Capturing Easy to capture Hard to capture
Objective Helps you verify the functionality of the software. Helps you to verify the performance of the software.
Area of focus Focus on user requirement Concentrates on the user’s expectation.
Documentation Describe what the product does Describes how the product works
Type of Testing Functional Testing like System, Integration, End to End, API testing, etc. Non-Functional Testing like Performance, Stress, Usability, Security testing, etc.
Test Execution Test Execution is done before non-functional testing. After the functional testing
Product Info Product Features Product Properties

Best practice of Functional Requirement

Important best practice for developing functional requirement document is as follows:

  • Do not combine two requirements into one. Keep the requirements granular.
  • You should make each requirement as complete and accurate as possible.
  • The document should draft all the technical requirements.
  • Map all requirements to the objectives and principles which contributes to successful software delivery
  • Elicit requirements using interviews, workshops and casual communications.
  • If there is any known, verified constraint which materially affects a requirement then it is a critical state that should be documented.
  • It is necessary that you document all the assumption in the document.

Mistakes While Creating a Functional Requirement

Here, are some common mistakes made while creating function requirement document:

  • Putting in unjustified extra information that may confuse developers
  • Not putting sufficient detail in the requirement document.
  • You add rules or examples, scoping statements or objectives anything except the requirement itself.
  • Left out a piece of important information that is an absolute must to fully, accurately, and definitively state the requirement.
  • Some professionals start to defend the requirements they have documented when the requirement is modified, instead of finding the correct truth.
  • Requirements which are not mapped to an objective or principle.


  • Explain functional requirements in Software Engineering: A Functional Requirements define a system or its components
  • Functional Requirements Document should contain Data handling logic and complete information about the workflows performed by the system
  • Functional requirements along with requirement analysis help identify missing requirements
  • Transaction corrections, adjustments, and cancellations, Business Rules, Certification Requirements, Reporting Requirements, Administrative functions, Authorization levels, Audit Tracking, External Interfaces, Historical Data management, Legal or Regulatory Requirements are various types of functional requirements
  • As a good practice do not combine two requirements into one. Keep the requirements granular.
  • Putting in unjustified extra information that may confuse developers should be avoided in the functional requirement document. To understand how these requirements translate into actual testing procedures, you may want to explore this guide on functional testing.