Non functional requirements

What are non functional requirements?

Non functional requirements describe how well a system should perform, behave, or operate.
They define quality attributes such as performance, security, scalability, usability, and reliability.

Unlike functional requirements, which explain what the system must do, non functional requirements describe the conditions under which the system must operate.

Why non functional requirements matter

• ensure the system works reliably under real world conditions
• influence architecture, infrastructure, and technology decisions
• help teams estimate effort more accurately
• prevent performance bottlenecks and scaling issues
• improve user satisfaction through better speed and usability
• reduce technical risks early in the project

Without clear non functional requirements, teams might build the correct features but still deliver an unsatisfactory product.

Key categories of non functional requirements

1. Performance

How fast the system should respond.

Examples:

• pages load in under 2 seconds
• API responses under 300 ms
• system supports 1000 requests per minute

2. Scalability

How the system handles growth in users or data.

Examples:

• support up to 50 thousand monthly active users
• handle traffic spikes during promotions

3. Security

Protection of data, access control, and compliance.

Examples:

• password hashing with industry standards
• role based access control for admins
• encryption of sensitive data

4. Usability

Ease of use for users.

Examples:

• mobile layout optimized for screens under 400 px
• accessibility meets WCAG 2.1 AA

5. Reliability

How consistently the system performs.

Examples:

• uptime of 99.9 percent
• automatic recovery from server failure

6. Maintainability

Ease of updating, fixing, or extending the system.

Examples:

• code follows agreed standards
• CI pipeline required for deployments

7. Compatibility

Support for browsers, devices, or platforms.

Examples:

• works on latest two versions of Chrome, Safari, Firefox

8. Compliance

Legal or industry standards.

Examples:

• GDPR compliance
• audit logging for financial records

Examples of non functional requirements in projects

• the system must handle up to 500 concurrent users
• average API response time under 250 ms
• the platform must support English and Spanish
• the application must work offline for up to 30 minutes
• accessibility level AA required
• scheduled downtime cannot exceed 2 hours per month

Functional vs non functional requirements

Aspect	Functional requirements	Non functional requirements
Focus	What system does	How system performs
Type	Features and actions	Quality attributes
Example	User can upload files	File uploads under 5 seconds
Used by	Dev, QA, design	DevOps, architects, QA

Both are needed for accurate scope, estimation, and delivery.

How to define clear non functional requirements

1. Use measurable metrics

Avoid vague statements like “fast” or “secure”.

2. Link to business goals

Performance or security often supports critical business needs.

3. Validate with technical leads

Architects and senior developers can identify feasibility.

4. Include non functional items in estimation

Performance, security, and accessibility often add real effort.

5. Prioritize

Not all quality attributes are equally important.

Non functional requirements in a document format

NON FUNCTIONAL REQUIREMENTS
Project: Logistics Platform
Version: 1.0

Performance
• dashboard loads in under 1.5 seconds
• API responses under 200 ms

Security
• two factor authentication for admin accounts
• all data encrypted at rest

Scalability
• support up to 20 thousand daily active users

Availability
• uptime target 99.9 percent

Accessibility
• interface meets WCAG 2.1 AA guidelines

Common mistakes

1. Requirements too vague
2. Mixing functional and non functional items
3. Ignoring performance until late in the project
4. No involvement from devops or architects
5. Not estimating non functional work separately
6. No success metrics

Best practices

• define non functional requirements during discovery
• involve both business and technical roles
• keep them measurable and testable
• revisit them after scope changes
• include them in acceptance criteria when relevant

FAQ

What are non functional requirements in software development?
Quality attributes that describe how the system should behave, such as performance, security, or reliability.

Why are non functional requirements important?
Because they ensure the system performs well under real conditions and avoids performance or security issues.

How do non functional requirements differ from functional requirements?
Functional requirements describe what the system does. Non functional requirements describe how well it must do it.

Who defines non functional requirements?
Usually technical leads, architects, QA, and product managers during discovery.

Can non functional requirements affect cost and timeline?
Yes. Performance, security, and scalability often require extra work and should be included in the estimate.