Functional and Non-Functional Requirements: Differences and Best Practices

by Tosho Trajanov

8 min read · Mar 15, 2021

The end goal of every project across all industries is the same: build a high-quality product that appeals to the customer.

But here's the catch:

In order to build a high-quality product, everyone in the process needs to understand their requirements clearly.

For instance, unclear functional requirements can lead to poorly defined scope. 

On the other hand, ignoring non-functional requirements can greatly lower the quality of the product. 

The truth is that delivering high-quality software is a delicate process that takes time, skills, and proper communication between different stakeholders. 

But what exactly is the difference between functional and non-functional requirements? How are they written? And most importantly, what are some of the best practices when writing your requirements?

Bear with me because I'm going to answer all of these questions in the post below. 

Let's get started! 

What Are Functional Requirements?

Functional requirements form the behavior of the system. If someone is giving you functional requirements for a project, they're giving you information on how the project's product is going to work.

These requirements describe the interactions between a system and its environment. 

Let's say that you already have a product that's out on the market, called Product 1.0. Now, your project is to work on Product 2.0.

The new features of the Product 2.0 would be functional requirements. The bugs that you will be fixing from Product 1.0 are also your functional requirements.

The new behavior you're building in the product is also a functional requirement. 

What Are Non-Functional Requirements?

Non-functional requirements, also known as quality attributes, describe the general software characteristics. 

The list of primary non-functional requirements includes:

  • Usability: the ability of a user to easily interact with a product
  • Reliability: the product should be secure enough to handle users' sensitive information
  • Performance: how a product reacts to user's actions 
  • Error handling: how the product handles errors 

For example, let's say that your Laravel developers are building a ride-hailing app. One implicit expectation from the app is to load as soon as possible. When users land on your app, they shouldn't wait 5 minutes for the app's content to load. 

What Is the Difference Between Functional and Non-Functional Requirements?

The major difference between functional and non-functional requirements is that functional requirements explain What the system does, while non-functional requirements explain How the system works.

Functional requirements are easier to define as they're driven by the business idea. They involve all the features of the project and the ways users engage with those features.

On the other hand, non-functional requirements are driven by experience. The best way to identify non-functional requirements is to analyze your product's performance and make it useful and convenient for users. 

To draw a line between functional and non-functional requirements, it'll be simpler to look at an example. We'll take Instagram as an example and assume how a development team would write their documentation.

Functional requirements:

  • User stories: Instagram users want to upload image and video content, write captions, and publish the update. They also want to react to content published by other accounts. 
  • Use cases: when a user opens their Instagram, the first page is the feed. Here, the user can interact with the content, leaving likes and comments. 
  • Additional functionality: push notifications, automated location tagging, automated expiration of user stories, photo filters, search bars, etc. 

Non-functional requirements:

  • Performance: content loading is a priority as users spend more time viewing content than uploading content.
  • Usability: users should be able to intuitively navigate between profiles and their subscriptions.
  • Reliability and scalability: the system should work well in low-latency conditions, showing media content with the smallest delay possible.
  • Safety: Instagram has detailed authentication protocols, photo upload, API integration, photo embedding, and encryption of direct messages.

How Are Functional and Non-Functional Requirements Written?

There are a few different ways to write functional and non-functional requirements. Here are the most common: 

Requirements Specification Document

This document is one way to formalize functional and non-functional requirements. It describes the functions and capabilities the product must provide. It can also contain constraints and assumptions.

The document is usually written in a textual form, although it's recommended it contains visual elements as well so that non-technical stakeholders understand the scope. 

A requirements specification document generally contains the following elements:

  • Purpose (definitions, system overview, and background)
  • Overall description (assumptions, constraints, business rules, and product vision)
  • Specific requirements (system attributes, functional requirements, database requirements)

Use Cases

Use case document is a document that describes the interactions between the system and the external users in a generalized, impartial way. It's a written description of how users will perform tasks in the system. It outlines a system's behavior as it responds to a request. Each use case contains a sequence of steps, starting with a user's goal and finishing when that goal is completed. 

User Stories 

User stories describe the functionality from the perspective of the end-user. They include what exactly the end-user wants the system to do. Agile teams organize user stories in a backlog, which is an ordered list of product functions. 

For example, a user story may be the following: 

As Jane, I want to invite my friends to join the app to experience it together. 

Functional Decomposition 

Functional decomposition is a document in which complex functionality is broken down into structural parts. This document aims to help simplify the development process early on and estimate project duration and costs

For example,

Reserve a room -> Log in -> Select hotel and date -> Confirm reservation

What Are Some of the Best Practices? 

Requirements should be clear and easy to understand.

Whether it's a use case or a functional decomposition, requirements should be written in a way that is clear and easy to understand. Keep in mind that some stakeholders won't be well-versed in the technological language, making the document as easy to understand as possible. Always choose the active voice over the passive and keep your sentences short.

Requirements should be written accurately.

Make sure your requirements document is written accurately, using consistent terminology and format. Avoid giving irrelevant information that may just confuse the reader. 

Use requirement imperatives correctly.

Many requirements documents contain imperatives such as "shall," "will," and "should." Some people will say that "shall" is a requirement, while others that "should" and "must" is a requirement. Your company should outline what each of these means and how to use them correctly when writing requirements. 

Don’t use weak phrases and subjective words.

For instance, if you write "something bad happened to the system," "something bad" can mean different things to different people. It's critical not to use these words. If you use them, make sure they're going to be interpreted in the same way by everyone.

Functional Requirement Non-functional Requirements
- Functional requirements form the behavior of the product.- Non-functional requirements describe the general software characteristics.
- They explain “What the system does.”- They explain “How the system works.”
- Helps you verify the functionality of the software.- Helps you verify the performance of the software.
- They are captured in use cases.- They are captured as a quality attribute.
- Easy to define.- Difficult to define.
- Focus on user requirement.- Focus on the user's expectation and experience.

- Examples (Netflix):

Users should be able to play any video by pushing the play button. The platform’s search bar should allow users to filter results by genres. The platform frequently updates the suggestions list by using data analytics.
- Examples (Netflix):

The interface should be desktop and mobile friendly. Every user should fill a form with personal details and provide financial data for paid subscriptions. The platform should comply with copyright laws.

Final Word

Knowing the difference between functional and non-functional requirements can be challenging. But understanding the difference can also set up the foundation for a successful project. 

Remember that both requirements are equally vital, despite the name one of them carries. After all, a website can meet its functional requirement to load in 30 seconds, but it will still be unusable. 

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FAQs

Q: What are functional and non-functional requirements?
The major difference between functional and non-functional requirements is that functional requirements explain What the system does, while non-functional requirements explain How the system works.
Q: What are examples of non functional requirements?
  • Usability
  • Reliability
  • Performance
  • Error handling
Q: What are examples of functional requirements?
  • Administrative functions
  • Authentication
  • External Interfaces
  • Certification Requirements
  • Reporting Requirements
  • Historical Data