Functional requirements define what a system must do: the specific features, actions, and behaviours that software must deliver to meet user and business needs. Non-functional requirements define how the system must perform: speed, security, scalability, reliability, and usability. Both are necessary. Understanding the distinction, and knowing when to prioritise one over the other, is one of the more consequential decisions engineering teams make during project scoping.
What Are Functional Requirements?
In software engineering, functional requirements describe what a system is supposed to do. They are generally expressed in terms of:
- User actions: Describing how users interact with the system, such as logging in, submitting forms, or accessing reports.
- System behaviour: Outlining how the system should respond to specific inputs or triggers, including data processing, database management, and interactions with other systems.
Functional requirements form the foundation of a project because they define the core capabilities the system must deliver to achieve its business objectives. For companies providing software engineering services, well-defined functional requirements are what allow engineers to build solutions that actually match what the client needs.
Examples of Functional Requirements
- User authentication with email and password login
- Real-time data updates across multiple devices
- File upload capabilities with file size restrictions
What Are Non-Functional Requirements?
While functional requirements define what the system should do, non-functional requirements define how the system performs those tasks. These requirements are equally important: they directly affect user experience, system quality, and operational reliability.
Non-functional requirements cover aspects such as:
- Performance: How fast should the system respond under various conditions?
- Security: What measures must be in place to protect sensitive data?
- Scalability: Can the system handle increased user loads as the business grows?
- Usability: How easy is it for users to navigate and interact with the system?
Non-functional requirements ensure that the software operates correctly in the real world, addressing concerns like system downtime, security vulnerabilities, and user experience degradation under load.
Examples of Non-Functional Requirements
- System must respond to user input within 2 seconds under normal load
- Data must be encrypted in transit and at rest
- The system must support 10,000 concurrent users without performance degradation
Key Differences Between Functional and Non-Functional Requirements
| Dimension | Functional Requirements | Non-Functional Requirements |
|---|---|---|
| Define | What the system must do | How the system must perform |
| Focus | Business objectives and user needs | System quality and user experience |
| Measured by | Test cases (does the feature work?) | Performance metrics (response time, uptime, compliance) |
| Priority timing | Early stages and MVP definition | Scale, post-MVP, and regulated environments |
| Examples | Login, search, payment processing | Speed, encryption, 99.9% uptime, SOC 2 compliance |
Balancing Functional and Non-Functional Requirements
When to Prioritise Functional Requirements
In the early stages of a project, functional requirements typically take precedence. Without clear definitions of what the software should do, it is impossible to build a working solution. Projects focused heavily on user-facing features, such as e-commerce platforms or mobile apps, start here. Examples:
- E-commerce platform: Functional requirements include product search, shopping cart management, and payment processing.
- Custom engineering project: Functional requirements cover specific workflows or integrations with third-party systems.
When to Prioritise Non-Functional Requirements
Non-functional requirements move to the front when the project scales beyond MVP or operates in a regulated environment:
- Banking application: Security and performance requirements around data encryption and transaction integrity are non-negotiable before launch, regardless of feature completeness.
- SaaS platforms: Scalability must be designed in from the start for platforms anticipating rapid user growth.
Three Common Challenges with Non-Functional Requirements
- Ambiguity. Requirements like "the system should be easy to use" are too vague to implement. Clear, measurable benchmarks are necessary: response time targets, uptime SLAs, and user error rates.
- Testing complexity. Verifying non-functional requirements often requires specialised environments. Scalability testing may involve simulating thousands of concurrent users to identify where the system degrades.
- Resource constraints. Optimising for performance or security demands significant development effort. Teams must balance these requirements against project timelines and budgets.
How Both Types of Requirements Shape the SDLC: Four Phases
- Requirement gathering. Functional requirements provide the roadmap for the software's capabilities. Non-functional requirements must also be captured at this stage, particularly where they influence architecture decisions. If high availability is a non-functional requirement, the team may select a particular infrastructure from the outset rather than retrofitting it later.
- Design and architecture. Functional requirements influence UI and UX design. Non-functional requirements guide architecture decisions: whether a microservices architecture is needed to ensure scalability, or whether specific compliance standards require data to be stored in a particular region.
- Development. Functional requirements translate directly into code and features. Non-functional requirements introduce additional work: optimising database queries, securing APIs, configuring caching layers, and building monitoring into the delivery pipeline.
- Testing. Functional testing verifies that the system works as expected using unit tests and integration tests. Non-functional testing requires performance testing, security audits, and load testing to confirm the system behaves correctly under stress.
Real-World Example: E-Commerce Platform
Functional requirements:
- Users can search for products by name, category, or brand
- Users can create accounts, add items to a cart, and complete secure payments
Non-functional requirements:
- Pages must load within 2 seconds for users in North America
- All user data including payment details must be encrypted in transit and at rest
- The system must handle peak traffic during major sales events without degradation
Functional requirements ensure users can complete their shopping journey. Non-functional requirements ensure the journey is fast, secure, and available when it needs to be. Both matter. Skipping either produces a system that either does the wrong thing reliably, or the right thing intermittently.
Defining Success with Both Requirement Types
Engineering teams that treat non-functional requirements as an afterthought consistently face performance regressions, security incidents, and costly architectural rework later in the lifecycle. Getting both right from the start is the more efficient path. See how Scrums.com engineering teams handle requirements definition as part of a structured discovery process.
Frequently Asked Questions
What is the difference between functional and non-functional requirements?
Functional requirements define what a system must do: specific features, actions, and behaviours. Non-functional requirements define how the system must perform: speed, security, scalability, and reliability. Both are necessary for a working production system.
Which type of requirement should be defined first?
Functional requirements are typically defined first because they establish what the system needs to do before you can determine how it needs to do it. However, non-functional requirements that affect architecture, such as compliance mandates or availability targets, should be captured early in the requirements process rather than retrofitted later.
What happens if non-functional requirements are ignored?
Systems built without non-functional requirements often perform well in development but fail in production: slow response times under load, security vulnerabilities, inability to scale, and poor user experience. Fixing these issues after the system is built is significantly more expensive than designing for them from the start.
Are non-functional requirements harder to test?
Generally, yes. Functional requirements can be validated with straightforward test cases: does the feature work or not? Non-functional requirements require specialised testing: load testing, penetration testing, uptime monitoring, and compliance audits. They often require dedicated tooling and environments to verify correctly.
