How to differentiate Functional and Non Functional Requirements

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Functional and Non Functional requirements represent the behavior and quality that the end-user expects from the final product. Features based on these requirements are counted as deliverables. These deliverables are needed to be delivered for the successful completion of the project. In addition, the main goal of any project is to frame all the requirements together and deliver them as a product that meets the end-users expectations.

In this blog, we are going to discuss the functional and non-functional requirements. At the end of this blog, you will be able to differentiate whether the requirement is functional or non-functional.

functional and non functional requirement gathering

What is a Functional Requirement?

The functional requirements describe the system’s functionality (i.e. it specifies “what the system should do”). Furthermore, these are the expectations that the end-user specifies as essential features that the system should provide.

In other words, we can say that functional requirements represent the behavior of function of the system when certain conditions are met. This behavior may be expressed as functions, services, or tasks that the system is required to perform. For example: To develop a login application, we have a requirement that the system should send an email to the manager when an employee is on leave or Late login. This requirement will be a functional requirement.

Some more examples that come under functional requirements:

  1. Certification requirements
  2. Audit tracking
  3. External interfaces
  4. Authentication
  5. Business rules
  6. Historical Data management
  7. Legal and Regulatory Requirements

Let’s have a quick look at key-point for functional requirements

  • They specify “What software system should do”
  • These are specified by the user
  • Helps to verify software’s functionality
  • These are easy to define
  • These are mandatory for system operations
  • Focus on user requirements

What is a Non-Functional Requirement?

A non-functional requirement specifies the quality attribute of a software system. These are the expectations that the end-user specifies as “how the system should work”. They are also termed non-behavioral requirements.

In other words, non-functional requirements are all about system usability. It focuses on the system’s operations that will perform rather than its behaviors.

Non-functional requirements might not be seen as important as functional requirements as they do not impact the functionality of the system. Though it plays a very important role in user experience while using the system thus it is also important.

Now let’s have a look at an example to understand how it affects the user experience. Consider that you are checking the online score for cricket, but the website takes more than 10 seconds to update the score. Thus this will give a bad user experience. As speed is the non-functional requirement (i.e. how fast the system performs certain activities). So, for a good user experience, we can focus on this requirement and make it update the score every 1 second.

Some more examples that come under non-functional requirements:

  1. Availability – Hours and location of operations.
  2. Speed – How fast the system performs certain activities.
  3. Usability – How easy the system is, to use for the customer or end-user.
  4. Reliability – How dependable the system is.
  5. Capacity – What the limits are of what the system can handle.
  6. Security – How are the system and its data protected against attacks.
  7. Portability and compatibility – Which hardware, operating systems, browsers, and their versions do the software run on?

Let’s take a quick look at key-point for non-functional requirements

  • It specifies “How system software should perform”.
  • It is specified by a technical team.
  • Helps to verify the performance of the software system.
  • These are hard to define than functional requirements.
  • These are not always important but are desired sometimes.
  • Focus on the user experience and expectations.
  • It is captured as a quality attribute of the system.

General recommendations to documenting Requirements

documentation of functional and non functional requirement

Functional Requirements

  • Do not combine two requirements into one.
  • You should make each requirement as complete and accurate as possible.
  • Draft all the technical requirements in the documents.
  • Map all requirements to the objectives and principles which contribute to successful software delivery.
  • Gather requirements using workshops, interviews, and casual communications.
  • You must document all the assumptions in the document.

Non-Functional Requirements (NFR)

  • Make them measurable and testable.
  • Do not combine two requirements into one.
  • Link NFR with business objectives.
  • Consider third-party limitations.
  • Consider architectural limitations.
  • Look for existing standards and guides.


Let us summarize the functional and non-functional requirements with the help of an example.
Let us consider that we have to develop an online patient-doctor appointment system. This system will help patients to book or cancel appointments online. Also, they can track appointments and place orders for online medicines.

For the following system, the functional requirements can be:

  1. Patients can log in/Sign Up into the system.
  2. Can book or cancel the appointment with the doctor.
  3. Order the online medicines.
  4. Doctors can update their availability.
  5. Track history using the patient portal.
  6. A patient can pay online fees.

Non-functional requirements can be:

  1. The system provides acknowledgment in just one second once the patient’s information is verified.
  2. It can support at least 2000 people at once.
  3. The system should be available all the time.

Written by 

Gaurav Kumar is an Intern in Knoldus practicing in Agile Studio. He thrives on difficulties and set objectives for himself on a regular basis. He always keeps himself in a learning state because learning and practicing is the only key to success.