Functional vs Non-Functional Requirements
Specify how well the product must work, not only what it does. · Execution & Communication · Lesson 59 · 3 min
Functional vs Non-Functional Requirements · 3 min
Situation
The report exists but arrives too late.
A team delivers a report that contains the right information. It takes fifteen minutes to open during a two-minute customer call.
The function is present, but the product fails in context. Performance, reliability, accessibility, and security can be central to whether a capability is useful.
Mental model
Behavior and quality constraints.
Functional requirements describe what the product does: generate a report, invite a teammate, update an address. Non-functional requirements describe qualities and constraints such as speed, availability, access protection, and usability with assistive technology.
The distinction helps uncover missing requirements. It should not imply that quality is secondary or optional.
Example
Replace vague adjectives with a context.
“Reports should be fast” is ambiguous. Define the report size, expected workload, relevant response-time measure, and threshold with engineering. For example, an agreed p95 target for a standard report under expected peak traffic.
For accessibility, describe keyboard operation, labels, focus handling, and relevant standards. “Accessible” needs validation, not a checkbox in the brief.
Failure case
Picking a number without understanding the cost.
A near-instant response target for an enormous report may require expensive precomputation or reduced freshness. The team needs to know whether the user truly needs immediate results.
A background report with clear status might satisfy the job at lower cost. Discuss the experience and the technical trade-off together.
PM decision
Negotiate the constraints before delivery.
Ask which quality requirements are essential for launch, how they will be measured, and who owns the checks. Define the degraded experience when the target cannot be met.
Include expected growth and important environments. A page that performs well on a fast office connection may fail the intended mobile use case.
Remember this
“It works” needs conditions.
Specify the relevant load, environment, quality bar, and failure behavior. A complete requirement says what happens and whether it remains useful under real conditions.