Functional and Non-functional Requirements

Functional requirements (FR)

Describe what the system must do: its functions, features, inputs, processing and outputs.

• Example (online shopping): The system shall allow a user to add items to a cart, place an order and receive an order confirmation.
• Written as "The system shall..."; tested by black-box functional tests.

Non-functional requirements (NFR)

Describe how well the system must work: quality attributes and constraints.

• Categories: performance, security, reliability, availability, usability, portability, maintainability, scalability, plus legal and standards constraints.
• Example: The system shall respond to a search within 2 seconds for 500 simultaneous users.

Comparison

Basis	Functional	Non-functional
Describes	What the system does	How the system performs (quality)
Focus	Features and behavior	Quality attributes and constraints
Source	User/business needs	Standards, constraints, quality goals
Verification	Functional testing	Performance, security, usability testing
If missing	System cannot do its job	System is unusable, slow or unsafe
Example	Login, generate invoice	Login in under 2 s, 99.9% uptime

Importance

Both must be recorded in the SRS. A system that meets all functional requirements but fails its NFRs (slow, insecure) is still a failure.