Functional vs. Nonfunctional Requirements: Explained

By Indeed Editorial Team

Published 31 May 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

Explaining the difference between functional and nonfunctional requirements can help software developers and systems engineers meet the expectations of their clients. If you're building a website, system, application or component, clarifying the functional and nonfunctional system requirements can keep you on the same timeline as the customer and help deliver the project in a timely fashion and in accordance with their standards. This article is going to define functional and nonfunctional requirements and show the differences between the two, with a look at the negative and positive aspects of each, with an explanation of the use-cases of these requirements and examples of each.

Understanding functional vs. nonfunctional requirements

Understanding the differences between functional vs. nonfunctional requirements is essential to keeping developers and their clients synchronised. Being able to prioritise functional and nonfunctional requirements can keep developers and engineers on target for their deadlines. Understanding which software requirements are most important to the customer can help with time management and ensure client satisfaction. Businesses can sort their requirements according to their function before the job begins. This can give their software developer a clear roadmap for the work.

Without the functional requirements, the system cannot fulfil its primary purpose. Many functional requirements have a series of nonfunctional requirements associated with them. Having a list of priorities can allow the developer to see which nonfunctional requirements are essential and which they can omit if there are unexpected budget or time constraints. If developers and businesses understand the differences between functional and nonfunctional requirements, they can achieve better timeline management and avoid any miscommunication or confusion.

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What are functional requirements?

Functional requirements are features of a system, application or site that are necessary for the entire product to function. If just one of the functional requirements of your system isn't operational, the system cannot perform successfully. This makes them integral to the overall functionality of the system. As a result, functional requirements define what your system can offer and what its use-cases are going to be. Most systems and applications have different functional requirements that are directly related to the services that the system can offer.

To use some examples from outside the world of software development, the functional requirement of a car is to drive. Similarly, the functional requirement of a helmet is to protect your head. Without meeting the functional requirements, the final product can't be fit for its purpose. In software development, not meeting the functional requirements can lead to faulty software. You can agree on the functional requirements of the software with your client beforehand. Meeting their expectations can help you meet deadlines, and it may lead to satisfied customers.

What are nonfunctional requirements?

Nonfunctional requirements are the processes used by a system, application or site to support the delivery of the functional requirements. Nonfunctional requirements are not necessarily essential for the system to fulfil its purpose. As a result, they don't define the primary function of a system or program. These nonfunctional requirements optimise the user experience and ensure the functional requirements are operating efficiently. In software development, the nonfunctional requirements often relate directly to the user interface (UI) and user experience (UX).

Continuing with our previous examples can illustrate this further. If the functional requirement of a car is to ‌drive, a nonfunctional requirement would be to have seats. While seats are not essential for the car's primary function, they provide a much better user experience when driving. The nonfunctional requirement of a helmet would be the strap that affixes it to your head. The helmet is protective on its own, but without the strap, the helmet may not function properly as a protective device.

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Uses of functional requirements

Depending on the type of program or software you are building, the functional requirements are unique. This means that there are many uses of functional requirements. As we've already seen, even something as simple as a bike helmet has a set of functional requirements. System requirements, such as a need for wireless connectivity or the ability to back up databases automatically, are common functional requirements seen by software developers. This also includes software and hardware specifications, such as processing speed and information transfer.

Functional requirements are also required to satisfy the client's goals for the application, site or system. Businesses may require a system to provide an online catalogue of products that allows users to place an order. They could also require the system to generate approval requests when a user submits information. If a company is required to send sales and viewing information to managers at the close of business, you could also be required to include separate authorisation levels for different members of staff.

Uses of nonfunctional requirements

The nonfunctional requirements of a system support the accessibility and usability of the functional requirements. As a result, there's a wide scope for the use of nonfunctional requirements when building software, applications and systems. User accessibility is one of the main uses of nonfunctional requirements. These accessibility requirements ensure that the user can easily navigate their way around the system and access the functional requirements. This could include the font style and size, alongside the design and positioning of links and buttons.

System reliability and expandability are also important uses of nonfunctional requirements. Ensuring that the system's reliability lives up to the needs and expectations of the user is vital to the success of the application. For example, some systems are required to be operational all year round, while some close ‌periodically. Your system being online and accessible can lead to a positive user experience and a happy client. If your client plans on growing their customer or user base in the future, the system can scale up to meet the higher demand.

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Functional and nonfunctional requirement documents

It's important for businesses and software developers to have a clear agreement on the functional and nonfunctional requirements of the project. With an agreement between the two parties, they can keep the project running on time and on budget. The best way to keep everyone in agreement is to create a functional and nonfunctional requirement document before the work begins. Once you've clearly defined the requirements, everyone from stakeholders to coders can ‌work towards the same goal with no disagreement over priorities and timelines.

Your document might contain text, diagrams, images or even prototypes. You can include whatever you need on the document to demonstrate the requirements if it benefits those involved in the project. Your requirement document lists the functional requirements first, before outlining the nonfunctional requirements. As some functional requirements need nonfunctional requirements to be completed to operate properly, it may benefit you to group these essential nonfunctional requirements at the top of your document.

Benefits of using a requirement document

Keeping an accurate and detailed functional and nonfunctional document can keep your project on time and on budget. Without a clear understanding of what your customer wants, there's no way to guarantee that the product is going to be satisfactory to the customer. In the worst-case scenario, not having a requirement document can lead to the failure of the project. Understanding what the client considers as functional requirements and nonfunctional requirements can also help you ‌prioritise your work and refine the scope if there are unexpected time or budget constraints.

Being able to refine ‌your work as the project develops is crucial to delivering on your client's expectations. As each functional and nonfunctional requirement can have a cost associated with it, you can ‌remain on budget by eliminating some of the less essential nonfunctional requirements. This is when listing nonfunctional requirements that are needed to support your functional requirements is especially useful. With a clear document, you can make sure that your work meets its goals, while still staying on time and within budget if the circumstances of the project change.

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