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Functional and Non-functional Requirements: The Comprehensive Guide

Let’s say you’ve decided to buy a car. It is going to be a black hatchback with a sporting pedigree. But have you thought about its engine, number of doors, and fuel consumption? Will it be a Volkswagen Polo or a Citroen? Should it be petrol or diesel power? You need to get to grips with lots of nuances to feel pleased with your choice.


Just like buying a car, mobile application development needs a magnifying glass to get your ducks in a row. A bird’s-eye view will help BAs and PMs come up with better product documentation on short notice.

On the flip side, if the software requirements still remain vague even during the development stage, teams would require more time and financial resources. On top of that, the project failure rate is typically higher if developers have to blunder their way along.

How can you fix that? Well, it’s simple. Make sure to delineate functional (FRs) and non functional requirements (NFRs) for your project.

This article will explain how to cut down the project’s cost, boost team performance and future-prove your project success. We’ll also look under the covers of FRs and NFRs and share some useful tips.

The Importance of Software Requirements

A successful beginning is half the battle. A clearly stated requirement is half the project’s success. FRs and NFRs also make sure that the dev team and stakeholders are on the same wavelength and picture the same result in their heads. The more ambiguity you have in the development process, the lower your chances are to deliver a high-quality product.

To back up these statements with supporting evidence, here are some surprising stats:

  • More than 60% of projects are more likely to have an outright failure due to an insufficient business analysis approach.
  • data suggests that poor business analysis, i.e. vague requirements and limited communication are the main culprits of most project failures.
  • Also, hazy requirements automatically result in a 60% increase in time and budget.
  • Finally, over 40% of the tech development funding for software, personnel, and outsourced services will be guzzled up by poor requirements.

Meticulous FR and NFRs in software development help the company reap the following benefits:

  • Clearly established roles, terms, and responsibilities. Accurate requirements simplify communication between the development teams and clients. Also, by charting out the comprehensive plan, you eliminate the risks of misunderstandings.
  • Reduced back-and-forth with a client. Upfront coordination with business analysts presupposes explicit requirements and reduced development time.
  • More accurate estimation. The more scrupulous your requirements are, the less inaccuracy you will have regarding your time and financial resources.
  • Mitigated risks. When the team and stakeholders articulate the project details already during the inception stage, it minimizes blunders before they wreak havoc.
  • More tangible projects. Precise requirements allow teams to foresee the future result and build a product well-suited to the client’s needs.

In sum, pre-defined requirements help teams and clients to find middle ground, stick to the funding and time frames, as well as deliver a high-quality result that meets or exceeds clients’ goals and expectations.

Now let’s have a closer look at project requirement types.

Classifying Requirements

To breathe life into your brilliant business idea, you need to consider the following points:

  • Business requirements created by business analysts refer to high-level business requirements and are used by upper and middle management.
  • Stakeholder requirements refer to user needs and can be added to the aforementioned business requirements. However, while business requirements are high-level ones, these are detailed needs from the user perspective.
  • Solution requirements describe particular features of the product, service, or result. They explain what needs should be done to achieve business and stakeholder requirements. They further fall into:
  • Functional requirements describe product, service, or result behavior.
  • Nonfunctional requirements describe environmental conditions for a viable product.

Drawing A Line

To make the separating wall between FRs and NFRs more palpable, let’ have a look at the comparison table below:

Parameters FRs NFRs
Requirement Significant Insignificant
Capturing type Use case Quality attribute.
End-goal Product feature Product properties
Capturing Effortless Hard
Target Software functionality verification Software performance verification
Focal area User requirement User’s anticipation and experience.
Documentation Outlines what the product does Outlines the product configuration
Product Info Product Features Product Properties

Functional Requirements

Functional requirements describe a list of functions that the system must accomplish. Also, they define how a system should behave regarding specific inputs and particular situations. Therefore, teams and stakeholders should meticulously lay NFRs out to avoid further misunderstandings.

FRs include, but are not limited to:

  • Business Rules
  • Authentication functions
  • Searching requirements
  • Audit Tracking
  • External Interfaces
  • Archiving
  • Compliance Requirements
  • Backup and recovery

If you’ve set up a Scrum team, they will typically present all FRs on paper or in visual form (or both).

The FRs may come in the following forms and shapes.

FRs specification document

FRs spec document outlines the requirements to be used by the software solution. It describes the operations and activities that a system must be able to perform.

FR specification document should include the following points:

  • The purpose of the software being developed
  • An overall software description
  • NFRs

Use cases

Use cases refer to a certain number of interaction sequences that a system performs to drive value results to one or more actors.

Three elements that a use case must contain:

  • Actor aka the user who interacts with a process
  • System, which is the process that’s needed to obtain the end goal
  • Goal, which describes the successful user outcome

User stories

User stories are a short description of business functionality from the user’s standpoint. They allow teams to break down particular product features into smaller pieces and present them on index cards, sticking notes, or specialized tools. Once the team defines user stories, they add them to a backlog and then rank them according to the client’s needs.

Functional decomposition

Functional decomposition is the process that is aimed at dissecting a complex process. Once devs fetch the smallest parts of a subsystem, they can analyze how to start coding those parts. This approach helps teams get a closer look at each small part of a project while capturing the greater image.

Mobile app prototypes

A mobile app prototype is a prelim visual model that looks like a functioning application with core design and functions. However, it doesn’t entail working code. Prototyping can start with sketches or a paper interface and grow into an interactive model that works on your phone.

Prototypes resemble your future app visually, but they are usually not functioning software. They lay the ground for the first functioning version of your app, also known as MVP.

Non-functional requirements

What is a non-functional requirement?

In simple words, these requirements describe the user’s needs for using the functionality. They define the general attributes of the final system. Some people don’t like this term, because it says what things are not rather than what they are.

Examples of simple NFRs include:


Usability is a set of procedures that allows you to understand the convenience of a product from the user’s standpoint. Usability is more like a measure of site or program interface friendliness, it helps to make the site understandable and intuitive for the user.

Legal or Regulatory Requirements

Legal or regulatory requirements describe all applicable laws, rules, orders, and regulations. If the software doesn’t comply with these requirements, it may lead to legal punishment and fines.


Software reliability refers to the probability that a product will fulfill the required task in a particular environment for a predefined number of input cases. To failure-free software operation, your team should debug the product to avoid poor safety and issues with system components.


Performance describes the way your product reacts when users use it in different scenarios. Low performance may result in a negative experience and undermine system safety.

Quality assurance

Quality assurance is a procedure to guarantee software that aims at enhancing the software development process.

As we’ve mentioned above, some NFRs don’t lie on the surface and can become a missing point for developers and clients. The main reasons for that include:

  • Personal knowledge. NFRs have a subjective nature, which means that team players can interpret them in diverse ways.
  • Integrated nature. The focal points of NFRs may not be compatible with each other, since they usually have a general impact on systems.
  • Unawareness as for the NFRs. Muddled wording, confounding definitions, and the nonappearance of an all-around acknowledged classification scheme make comprehension of NFRs an uphill struggle.
  • Assuming that it’s universal knowledge. During the inception stage, both the stakeholders and the team might leave non-functional requirements unattended since they don’t make sense from a business point.

Nonetheless, non-functional requirements play a critical role in delivering a high-quality, trailblazing, and successful product.

How to define NFRs

To outline the most non-functional requirements, you should:

  • Break all NFRs into three classes: operation, revision, and transition. This will give you a sense of an outlined classification and provide a common ground for discussion.
  • Prepare for requirements elicitation by identifying a list of questions. It will boost team performance and pare down on time when getting ready for elicitation interviews and workshops.
  • Ensure effective communication with the dev team during the requirements definition to find a common ground.
  • Recycle the requirements already defined for other systems since software systems share similarities anyway when it comes to NFRs.
  • Utilize testing automation tools such as TestingWhiz, Ranorex, and Sahi. Such tools account for faster feedback cycles, bring faster validation for development phases, and expose more NFRs.

The Bottom Line

Software projects may not come to fruition for a lot of reasons. Vague requirements are certainly one of the common stumbling blocks that prevent projects from thriving. By defining both FR and NFRs, you can cut down development expenses and speed up the whole process.

Whereas functional requirements have to do with the functionality of the system, non-functional requirements refer to systems with non-functional properties, i.e. system qualities. Unlike FRs, NFRs do not always have clear satisfaction criteria, thus being impalpable. Nevertheless, companies and clients have to take a step further to get at the heart of the matter.