Suppose you were buying a motorcycle. What do you expect? You may desire it to go up to 170 miles per hour without failing. You may want to have the ability to add a sidecar or add luggage space with a trailer. Safety features matter too. Although these considerations don’t bear a direct relationship to the motorcycle’s intended purpose, moving an individual from point A to point B, they quite clearly affect the overall experience. In the same manner, software products also possess nonfunctional requirements. Whether a website, a mobile app, or a desktop platform, software must satisfy specific quality criteria to achieve user expectations.

nonfunctional requirements

What Are Nonfunctional Requirements?

Nonfunctional requirements (NFRs) specify a system’s operation capabilities and limitations. They tell us how well something operates as opposed to what it does. These requirements comprise qualities like speed, security, reliability, and data integrity.

Frequently termed as quality attributes or software quality requirements, NFRs constitute one group of software requirements. To understand them properly, it is worth differentiating them from functional requirements.

Functional vs. Nonfunctional Requirements

Functional as well as nonfunctional requirements describe attributes a software product should possess in order to fulfill business and stakeholder requirements. But their emphasis is different.

Functional requirements specify what a software product should do. For instance, in a messaging program, a functional requirement might be:

“A user should be able to edit messages once they have been sent.”

Nonfunctional requirements specify how the system should perform. For instance, for the same messaging program, an NFR might be:

“The message should be refreshed for all users within a chat within 0.1 seconds, if all users are online with an LTE connection or higher.”

Both categories of requirements are normally addressed in a Software Requirements Specification (SRS) or a Product Requirements Document (PRD). Both these documents state the required functionalities, constraints, and assumptions for the software.

Now that we have clarified the distinction, let’s look at the different categories of nonfunctional requirements.

Types and Examples of Nonfunctional Requirements

There are a number of categories of nonfunctional requirements that specify various facets of system behavior.

Performance requirements specify how fast a system reacts to user input. This encompasses page loading time, data retrieval speed, and transaction processing speed.
Scalability is the ability of a system to deal with growing workloads. As the number of users increases, the system should be able to sustain consistent performance.
Portability characterizes the software’s capability to run on varied hardware, operating systems, and browsers.
Compatibility ensures that the software does not clash with other applications or processes it communicates with.
Reliability quantifies the occurrence of system failure and frequency of critical faults.
Maintainability characterizes the speed and effectiveness with which a system can be fixed once issues occur.
Availability defines the duration for which the system will continue to work and the level of acceptable downtime.
Security is concerned with safeguarding the system and data against unauthorized access, cyber-attacks, and vulnerabilities.
Usability specifies how user-friendly and easy to use the system is for end users.

All of these nonfunctional requirements are important in providing a software product that satisfies performance, security, and usability requirements.

Performance Requirements

Performance requirements define how fast a system reacts to user inputs under a specific workload. It is an important part of software quality, guaranteeing the system achieves performance levels as anticipated.

Examples of Performance Requirements

The landing page should handle 5,000 users per hour and offer a response time of six seconds or less on a Chrome desktop browser over an LTE network.
Product search results need to be displayed within three seconds for 90 percent of searches in regular circumstances.
Data streams need to be processed at a rate of at least one million records per minute from more than one source without loss of data.
A real-time analytics dashboard needs to refresh and show updated information within ten seconds of receiving new input.

Best Practices for Defining Performance Requirements

Start with Google’s web performance recommendations. Google considers multiple factors, such as connection type and device performance, when ranking pages. Tools like Google PageSpeed Insights provide benchmarks for web page performance, which are useful for defining response time requirements.

Consider established response time metrics. Jakob Nielsen’s response time guidelines highlight three key limits:

1 second is the threshold for an instantaneous system response.
1 second is when users notice a delay, but their thought process remains uninterrupted.
10 seconds is the point at which users lose attention and engagement.

Studies indicate that approximately 55 percent of visitors leave sites which load more than three seconds. Performance has a direct correlation to user retention as well as to conversion.

Designate the measuring scenario. Set whether or not the performance indicator includes browser render time or simply backend response time. In cases where varied types of content get loaded in variable speeds, establish individual benchmarks for text, image, and video.

Establish workload conditions for testing. Performance will differ according to traffic volumes. For instance, throughout the rush hour, a system can have 5,000 concurrent users, whereas at night, it can be 1,000 users. Well document which scenarios the performance benchmarks hold.

Remove third-party dependencies. If data is retrieved from an external API, developers cannot control performance outside of their own infrastructure. Measurement of performance needs to be restricted to internal system operations.

Recognize architectural limitations. Legacy applications or enterprise-level solutions might have performance caps due to technical debt. Performance optimization in such scenarios can be more about architectural revamps than incremental changes.

Scalability Requirements

Scalability defines the highest workloads a system can handle while maintaining performance and usability. It reflects the system’s ability to accommodate growth in data volume and user load.

There are two main approaches to scalability:

Horizontal scaling, which involves adding more machines to the server pool.
Vertical scaling, which involves increasing CPU and RAM on existing machines.

The main goal of scalability requirements is that the system is stable and runs efficiently despite growing users, data, business processes, or other modules.

Examples of Scalability Requirements

The site has to handle one million simultaneous visitors with optimal performance.
The ERP system is capable of expanding data processing and storage capacity to match a tenfold transaction growth in five years.
The real-time location system should be able to scale between 10,000 and 500,000 IoT devices without losing data.
The eCommerce platform should support a 300 percent spike in traffic during holidays and other shopping frenzies without performance impact.

How to Address Scalability Requirements

Specify scalability expectations from a business standpoint by evaluating existing workloads and extrapolating future expansion. Account for industry details, including user load for eCommerce or data volume for banking apps. Measure requirements using concrete metrics such as concurrent user numbers, volume of data, and transaction volumes. Make sure that scalability objectives are practical according to projected growth patterns and peak loads.

Portability Requirements

Portability dictates whether a system or subsystem will be able to operate on diverse environments. These include hardware, software, and platform portability, where operations are smooth in multiple systems.

Examples of Portability Requirements

The software should operate predictably on all kinds of mobile devices with various screen sizes and resolutions.
The program needs to execute unchanged on Windows, macOS, and Linux.
The program for Windows 10 should function without behavior changes on Windows 11.
The web application should be entirely functional on popular browsers like Chrome, Firefox, Safari, and Edge.

How to Approach Portability Requirements

Portability requirements must be derived from market research, field studies, and analytics reports of the most commonly used platforms. Google Analytics is one tool that can assist in identifying device and browser preference. A complete set of portability requirements must contain supported operating systems, browsers, network details, and hardware limitations.

Compatibility Requirements

Compatibility ensures that a system will run as expected with other applications, software elements, or outside systems. It also provides interoperability, the ability to exchange data gracefully with other systems.

Incompatibility may lead to performance problems, errors, or loss of data. Compatibility requirements are particularly crucial when implementing third-party software or legacy systems.

Examples of Compatibility Requirements

The iOS app should be compatible with iPhone devices using OS versions 15, 16, and 17.
Healthcare data management software should be compatible with different EHR systems to enable secure exchange of patient data.
A video conferencing application should be compatible with webcams, microphones, and speakers from different manufacturers.
The ERP system should also provide compatibility with existing software in order to have a smooth data interchange.
Banking software should have the ability to import and export financial data in formats like CSV, XLSX, and QIF.

How to Approach Compatibility Requirements

Identify target systems the software needs to interface with. Establish third-party application compatibility, such as expected levels of performance. Address cross-platform, cross-browser, and mobile-responsive compatibility for compliance with industry standards.

Reliability Requirements

Reliability quantifies the probability of a system executing without failure within a specified time frame. It is often stated as a probability percentage, reflecting the likelihood of the system operating without defects under normal circumstances.

Examples of Reliability Requirements

The system should never fail in 95 percent of usage scenarios within a one-month timeframe.
All money transactions need to be 100 percent correct to guarantee data integrity at all times.
The system ought to recover from faults without losing data or processing it incorrectly.

How to Approach Reliability Requirements

Establish critical failure modes and normal usage conditions. A method is to monitor the rate of critical bugs discovered in production over time. Another way is to measure the mean time between failures (MTBF) as an estimate of system stability. Reliability standards should be defined from test results and actual field performance measurements.

Maintainability Requirements

Maintainability refers to the speed at which a system or its parts can be repaired, upgraded, or modified to accommodate changing needs. It is typically quantified by the mean time to restore the system (MTTRS), which is the estimated time to correct faults.

Examples of Maintainability Requirements

The system should be restored within 10 minutes after a failure, including all corrective maintenance and downtime.
The video streaming service must adapt video quality automatically according to user internet speed in order to prevent buffering.

How to Approach Maintainability Requirements

Assess the modularity of the system so that modification of one piece has less effect on others. Think about how long the software is supposed to last—if it is going to be around for a while, investing in maintainability is vital. If it is an MVP, then big maintainability investment may not be required in the early phases.

Availability Requirements

Availability quantifies how frequently the system is available and running. It is normally measured as a percentage of uptime within a certain timeframe.

Examples of Availability Requirements

The web dashboard should be accessible to users in the United States 99.98 percent of the time during working hours.
The application should run consistently on devices and operating systems with a 99 percent success rate.
The video streaming service has to provide 99.8 percent uninterrupted playback in normal network conditions.

How to Address Availability Requirements

Begin by identifying business downtime impact. If the system is not able to be down for more than a specific percentage of time, define that as a threshold. Look at the monetary losses or operations disruption resulting from system unavailability.

Identify whether availability needs to be met across the entire system or particular parts. For example, a payment system will need nearly flawless availability, whereas a marketing dashboard can be less demanding. Identify various load conditions to determine availability expectations during both normal and heavy workloads.

Security Requirements

Security requirements stipulate the way in which a system prevents attacks, data exposure, and access by unauthorized entities. Functional requirements detail how security is accomplished and follow industry rules and compliance standards, whereas nonfunctional requirements detail the entire security landscape.

Examples of Security Requirements

The payment system processing has to be PCI DSS compliant if it has to provide secure payments.
Clinical software has to be HIPAA and GDPR compliant for guarding patient information.
Cloud data centers have to be ISO 27001 security certification compliant.

Steps to Fulfill Security Needs

Determine the particular threat that your system needs to guard against, i.e., unauthorized entry, malware, or data breach. Specify under what circumstances these dangers are most likely to arise and what needs to be put in place.
Expand security-related nonfunctional specifications into functional ones. This includes authentication and authorization controls that control who can read, modify, or delete data.
Consider industry security standards within your industry. If compliance is necessary with specific regulations or guidelines, security requirements should explicitly specify these requirements.

Usability Requirements

Usability is a description of how easy, efficient, and straightforward a system is. It ensures that the users can operate and interact with the software without frustration or confusion.

The Nielsen Norman Group defines usability by five primary criteria:

Learnability – How quickly users can learn and use the system.
Efficiency – How efficiently users can accomplish their goals.
Memorability – To what extent users can easily come back to the interface after an interruption.
Errors – With what frequency users encounter errors and how easily they can recover.
Satisfaction – How pleasant the system is to use.

Examples of Usability Requirements

The error rate of submitting the payment form should not exceed 10 percent.
The user must be able to locate a product in three clicks from the home page.
Mute, video toggle, and screen share controls are primary controls and must be available during video calls.
The most-used features must be accessed through least navigation.

How to Approach Usability Requirements

Perform usability testing to assess users’ interaction with the system. Establish a baseline usability rating in the event that the product already exists and determine areas for improvement.
Study competitors’ products in an attempt to establish benchmark usability requirements. In the event that there is no prototype, usability testing on cutting-edge applications can serve as quantifiable usability targets.
Establish usability measures in terms of key performance indicators (KPIs). Where one of the main business objectives is rapid product exploration, usability must be developed to reduce navigation steps.
Test usability on the prototype level instead of waiting for the final product. This enables making improvements prior to launch, providing a seamless user experience.

How to Document Nonfunctional Requirements: Best Practices

Define Requirements for Specific System Components

NFRs should be implemented where they will do the most good. A case in point is that performance requirements on a seldom-used admin page can be avoided. Prioritize user-facing systems and high-level workflows.

Quantify and Test NFRs

In order to ensure a system satisfies its nonfunctional constraints, requirements must be measurable. Specify precise measurement units, testing methods, and success/failure criteria.

Rather than, “The system should be quick,” say, “The system must respond to user input in two seconds.”

Work with Stakeholders

Work with various groups in writing NFRs. Market research aligns usability expectations, investor feedback aligns business, and engineer feedback supplies feasibility.

Align NFRs with Business Goals

A one-minute difference in system availability may not be that important to revenues but would take weeks of extra engineering to achieve higher availability levels. Make good trade-offs between business requirements and development effort.

Take Third-Party and Architectural Constraints into Account

If the response of a third-party API is slow, system performance requirements must tolerate the limitation of this particular API. Legacy systems can demand significant refactoring in order to satisfy new nonfunctional requirements.

Adhere to Set Standards and Guidelines

Follow set best practices, compliance frameworks, and platform standards. For example, iOS and Android maintain strict application performance and user experience guidelines which should inform the development of mobile applications.

Monitor and Update NFRs

Since technology and business requirements evolve with time, review non-functional requirements from time to time. Revise them with respect to new market trends, technical innovations, or shifting business objectives.

Final Thoughts

Security and usability are critical aspects of software development, where software is secure, easy to use, and industry-standard compliant. Thoroughly documented nonfunctional requirements bring simplicity, consistency, and measurable objectives, and teams can use them to develop scalable, dependable, and sustainable software.

By making clear, testable, and achievable nonfunctional requirements, organizations can develop high-quality software that not only performs well but also provides better performance, security, and user experience.

Nonfunctional Requirements in Software Engineering: Examples, Types, Best Practices