Functional Requirements vs Non-Functional Requirements

Aspect	Functional Requirements	Non-Functional Requirements
Definition	What the system must do — specific behaviors or functions.	How the system should be — qualities or constraints of the system.
Focus	Features, tasks, and business logic.	Performance, usability, reliability, security, etc.
Examples	- User login/logout

• Create, read, update, delete (CRUD) data
• Generate reports
• Process payments | - System must respond within 2 seconds
• 99.9% uptime availability
• Data must be encrypted
• Support 1000 concurrent users | | Measured By | Functional correctness, completeness. | Performance metrics, SLA, security audits, etc. | | Impact | Directly impacts what the system does for users. | Impacts user experience, system stability, and compliance. | | Documented As | Use cases, user stories, feature lists. | Performance requirements, security requirements, compliance standards. | | Verification | Functional testing, user acceptance testing. | Load testing, security testing, usability testing. |

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Real-life analogy:

• Functional: "The elevator must go up and down when a button is pressed."
• Non-functional: "The elevator must arrive within 10 seconds, support up to 15 people, and have safety features." !