Non-Functional Requirements Explained

When designing any engineered system, you must define what the system should do and how it should perform. This distinction is captured in the separation between functional and non-functional requirements. While functional requirements outline specific tasks or capabilities, non-functional requirements (NFRs) define the conditions under which the system must operate.

In practice, this means a bridge must not only support vehicles (functional) but also withstand wind loads, require minimal maintenance, and comply with safety codes (non-functional). In systems engineering, these quality-related criteria are critical for ensuring performance, safety, and stakeholder satisfaction.

Defining non-functional requirements

Non-functional requirements describe the qualities or attributes of a system. These can include performance, reliability, safety, maintainability, usability, and compliance. Unlike functional requirements, which are typically framed as actions or behaviors, non-functional requirements serve as constraints or conditions the entire system must fulfill.

In general engineering contexts, NFRs are essential for verifying that the system operates reliably and predictably under specified conditions and over its intended life cycle.

Key categories of non-functional requirements

Across engineering disciplines, non-functional requirements typically include the following categories:

• Performance: Describes how well the system operates under specific conditions. For example, a power plant might be required to maintain output within 5% of its rated capacity under varying load demands.
• Reliability: Measures the system’s ability to operate without failure over a defined period. In aerospace, a navigation system might need to demonstrate a mean time between failures (MTBF) of 10,000 hours.
• Safety: Defines the system’s ability to avoid hazardous conditions. For example, an autonomous vehicle must detect obstacles and execute emergency stops within milliseconds.
• Maintainability: Concerns how easily a system can be inspected, repaired, or upgraded. A wind turbine might require a modular design to reduce downtime during maintenance.
• Usability: Focuses on how easily humans can interact with the system. Control panels in industrial environments are often subject to ergonomic assessments to ensure operator safety and efficiency.
• Compliance and standards adherence: Ensures the system meets regulatory or industry-specific standards. For example, in the space industry, products must comply with ECSS or ISO standards.
• Scalability and adaptability: Particularly relevant in infrastructure and industrial systems, this refers to the system’s capacity to handle increased loads or be modified for future needs.

Why non-functional requirements matter

In engineering practice, non-functional requirements are crucial for:

• Risk management: They help identify and mitigate potential hazards before deployment.
• Cost control: They guide design decisions that influence long-term operational costs and resource allocation.
• Lifecycle performance: NFRs ensure that systems continue to meet expectations through wear, environmental changes, and evolving user needs.
• Stakeholder confidence: Regulatory bodies, clients, and end-users rely on compliance with NFRs as evidence of a system’s fitness for purpose.

Best practices for managing non-functional requirements

To manage non-functional requirements effectively across engineering disciplines, consider the following practices:

• Early integration: Embed NFRs into the requirements analysis phase, not as afterthoughts. This supports better design trade-offs and avoids costly redesigns. When you have created your functional requirements, make your non-functional requirements right after.
• Quantification and measurability: Express NFRs in specific, testable terms. Instead of “the system should be fast,” use “the system must respond within 250 milliseconds.”
• Stakeholder involvement: Collaborate with all relevant stakeholders. These could include regulators, operators, and maintenance teams. It ensures all quality expectations are captured.
• Traceability: Maintain clear links between NFRs and design elements, test cases, and validation reports. This ensures compliance throughout the system lifecycle.
• Review and revision: As projects evolve, reassess NFRs to reflect changes in technology, operating environments, or stakeholder needs. Ensure you have a proper change management process in place.

From quality to compliance: The role of NFRs in systems success

In every engineering project, non-functional requirements act as the backbone of system integrity and operational success. They guide engineers from their goals to performance, cost, usability, and safety. They also form the basis for evaluation long after the system is deployed.

Whether designing infrastructure, machinery, or digital platforms, integrating NFRs from the outset ensures that the final product is not only functional but also resilient, efficient, and aligned with stakeholder expectations.

For a comprehensive solution to managing both functional and non-functional requirements, consider exploring our Requirements Management Solution.